The Principles of
Readability

By
William H. DuBay

Impact Information, 126 E. 18th Street, #C204, Costa Mesa, CA 92627, (949) 631-3309

Copyright
The Principles of Readability
25 August 2004
 2004 William H. DuBay. All Rights Reserved.

Abstract
The Principles of Readability gives a short history of literacy studies in the U.S.
and a short history of research in readability and the readability formulas.

Readers' Comments
Please send all comments and suggestions regarding this document to:William DuBay
Impact Information
126 E. 18th Street, #C204
Costa Mesa, CA 92627
Phone: (949) 631-3309
Email: bdubay@impact-information.com
Website: http://www.impact-information.com

Copyright © 2004 William H. DuBay

Page ii

Contents
Introduction .......................................................................................................... 1
Guidelines For Readability.......................................................................... 2
The readability formulas.............................................................................. 2
Are the readability formulas a problem? ..................................................... 2
What is readability?..................................................................................... 3Content ........................................................................................................ 3
The Adult Literacy Studies................................................................................... 4
Grading the reading skills of students.......................................................... 4
Grading adult readers .................................................................................. 4
U.S. military literacy surveys—reading on the job...................................... 4
U.S. civilian literacy surveys ....................................................................... 6
Challenges for technical communicators ..................................................... 9
The Classic Readability Studies ......................................................................... 10
L. A. Sherman and the statistical analysis of literature .............................. 10
Vocabulary-Frequency Lists...................................................................... 11
The Classic Readability Formulas ............................................................. 13
The New Readability Studies ............................................................................. 25
A Community of Scholars ......................................................................... 26
The Cloze Test .......................................................................................... 27
Reading Ability, Prior Knowledge, Interest, and Motivation .................... 28
Reading Performance ................................................................................ 30
The Measurement of Content .................................................................... 31
Text Leveling ............................................................................................ 35
Producing and Transforming Text............................................................. 37
The New Readability Formulas ................................................................. 43
Formula Applications ................................................................................ 55
Using the Formulas.................................................................................... 56
Conclusion.......................................................................................................... 57
References .......................................................................................................... 59
Biosketch............................................................................................................ 72

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The Principles of Readability

Copyright © 2004 William H. DuBay

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RESEARCH

Summary
Over 80 years of research and testing havecontributed to the worldwide use in many
languages of the readability formulas. They help
us improve the text on the level of words andsentences, the first causes of reading difficulty.

The principles of readability are in
every style manual. Readability formulas
are in every word processor. What is
missing is the research and theory on which they stand.

The Principles of Readability
By William H. DuBay

Introduction
In 1998, traffic accidents caused 46 percent of all accidental deaths of infants
and children aged 1 to 14 (National Center for Health Statistics, 2000). One
study (Johnston et al. 1994) showed that the single strongest risk factor for injury
in a traffic accident is the improper use of child-safety seats. Another study(Kahane 1986) showed that, when correctly used, child safety seats reduce therisk of fatal injury by 71 percent and hospitalization by 67 percent.
To be effective, however, the seats must be installed correctly. Other studies,
showed that 79 to 94 percent of car seats are used improperly (National Highway
Traffic Safety Administration 1996, Decina and Knoebel 1997, Lane et al.
2000).
Public-health specialists Dr. Mark Wegner and Deborah Girasek (2003)
suspected that poor comprehension of the installation instructions might
contribute to this problem. They looked into the readability of the instructions
and published their findings in the medical journal Pediatrics. The story wascovered widely in the media.
The authors referred to the National Adult Literacy Study (National Center for
Educational Statistics, 1993), which states the average adult in the U.S. reads at
the 7th grade level. They also cited experts in health literacy who recommend thatmaterials for the public be written at the fifth or sixth-grade reading level (Doaket al., 1996; Weiss and Coyne, 1997).
Their study found that the average reading level of the 107 instructions theyexamined was the 10th grade, too difficult for 80 percent adult readers in the U.S.
When texts exceed the reading ability of readers, they usually stop reading. The
authors did not address the design, completeness, or the organization of the
instructions. They did not say that the instructions were badly written. Armedwith the SMOG readability formula, they found the instructions were written at
the wrong grade level. You can be sure the manufacturers of the car safety seats
are scrambling to re-write their instructions.

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The Principles of Readability

Guidelines For Readability
In works about technical communication, we are often told how to avoid suchproblems. For example, JoAnn Hackos and Dawn Stephens in Standards forOnline Communication (1997) ask us to “conform to accepted style standards.”
They explain:
Many experts, through much research, have compiled golden rules of
documentation writing. These rules apply regardless of medium:•Use short, simple, familiar words•

Avoid jargon.

•

Use culture-and-gender-neutral language.

•

Use correct grammar, punctuation, and spelling.

•

Use simple sentences, active voice, and present tense.

•

Begin instructions in the imperative mode by starting sentences withan action verb.

•

Use simple graphic elements such as bulleted lists and numberedsteps to make information visually accessible.

For more suggestions, we recommend referring to one of many
excellent books on writing style, especially technical style.
We all know of technical publications that do not follow these guidelines and areread only by a small fraction of the potential readership. One reason may be thatthe writers are not familiar with the background and research of these guidelines.
This paper looks most carefully at two of the most important elements ofcommunication, the reading skills of the audience and the readability of the text.

The readability formulas
In the 1920s, educators discovered a way to use vocabulary difficulty andsentence length to predict the difficulty level of a text. They embedded thismethod in readability formulas, which have proven their worth in over 80 years
of application.
Progress and research on the formulas was something of a secret until the 1950s.
Writers like Rudolf Flesch, George Klare, Edgar Dale, and Jeanne Chall brought
the formulas and the research supporting them to the marketplace. The formulas
were widely used in journalism, research, health care, law, insurance, andindustry. The U.S. military developed its own set of formulas for technicaltraining materials.
By the 1980s, there were 200 formulas and over a thousand studies published on
the readability formulas attesting to their strong theoretical and statistical
validity.

Are the readability formulas a problem?
In spite of the success of the readability formulas, they were always the center ofcontroversy. When the “plain language” movement in the 1960s resulted inlegislation requiring plain language in public and commercial documents a
number of articles attacked the use of readability formulas. They had titles like,“Readability: A Postscript” (Manzo 1970), “Readability: Have we gone too far?”(Maxwell 1978), “Readability is a Four-letter Word” (Selzer 1981), “WhyReadability Formulas Fail” (Bruce et al. 1981), “Readability Formulas: Second
Looks, Second Thoughts“ (Lange 1982), “Readability Formulas: What’s theUse?” (Duffy 1985) and “Last Rites for Readability Formulas in TechnicalCommunication” (Connaster 1999).
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The Principles of Readability

Many of the critics were honestly concerned about the limitations of the formulas
and some of them offered alternatives such as usability testing. Although thealternatives are useful and even necessary, they fail to do what the formulas do:provide an objective prediction of text difficulty.
Although the concerns of the formula critics have been amply addressed
elsewhere (Chall 1984, Benson 1984-1985, Fry 1989b, Dale and Chall 1995,
Klare 2000), we will examine them again in some detail, with a special regard
for the needs of technical communication.
The purpose of this article is to very briefly review the landmark studies onreadability and the controversy regarding the formulas. I will be happy if you
learn something of the background of the formulas, what they are good for, andwhat they are not. That knowledge will give you greater confidence and method
in tailoring your text for a specific audience.

What is readability?
Readability is what makes some texts easier to read than others. It is often
confused with legibility, which concerns typeface and layout.
George Klare (1963) defines readability as “the ease of understanding orcomprehension due to the style of writing.” This definition focuses on writing
style as separate from issues such as content, coherence, and organization. In asimilar manner, Gretchen Hargis and her colleagues at IBM (1998) state thatreadability, the “ease of reading words and sentences,” is an attribute of clarity.
The creator of the SMOG readability formula G. Harry McLaughlin (1969)
defines readability as: “the degree to which a given class of people find certain
reading matter compelling and comprehensible.” This definition stresses theinteraction between the text and a class of readers of known characteristics suchas reading skill, prior knowledge, and motivation.
Edgar Dale and Jeanne Chall’s (1949) definition may be the mostcomprehensive: “The sum total (including all the interactions) of all thoseelements within a given piece of printed material that affect the success a groupof readers have with it. The success is the extent to which they understand it,
read it at an optimal speed, and find it interesting.”

Content
Beginning early in the last century in the U.S., studies of the reading ability ofadults and the readability of texts developed in tandem. Our subject matter falls
under these headings:
The Adult Literacy Studies These studies discovered great differences in
the reading skills of adults in the U.S. and their implications for society.
The Classic Readability Studies This section looks at the early readability
studies, which started in the late 19th century and concluded in the
1940s, with the publication of the popular Flesch and Dale-Chall
formulas. During this period, publishers, educators, and teachers wereconcerned with finding practical methods to match texts to the skills of
readers, both students and adults.
The New Readability Studies Beginning in the 1950s, new developments
transformed the study of readability, including a new test of reading
comprehension and the contributions of linguistics and cognitive
psychology. Researchers explored how the reader’s interest, motivation,and prior knowledge affect readability. These studies in turn stimulated
the creation of new and more accurate formulas.

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The Principles of Readability

The Adult Literacy Studies
Grading the reading skills of students
Before the mid-19th century, schools in the U.S. did not group students according
to grade. Students learned from books that their families had, often Bibles andhornbooks. American educator Horace Mann, who had studied the supervision,
graded classes, and well-articulated classes of Prussian schools, struggled tobring those reforms to America.
It was not until 1847 that the first graded school opened in Boston with a seriesof books prepared for each grade. Educators found that students learn reading insteps, and they learn best with materials written for their current reading level.
Since then, grouping by grades has functioned as an instructional process that
continues from the first year of school through high school and beyond.
Although reading standards were set for each grade, we know that not all
students in the same class read at the same level. A 7th-grade teacher, forexample can typically face a classroom of students with reading ability from the
2nd to the 12th grade. Good teaching practice has long separated students in thesame class by reading ability for separate instruction (Betts 1946, Barr andDreeben 1984).
Educators promoted the target reading levels for each class with the use ofstandardized reading tests. William A. McCall and Lelah Crabbs (1926) of the
Teachers College of Columbia University published Standard Test Lessons in
Reading. Revised in 1950, 1961, and 1979, these tests became an importantmeasure of the reading ability of students in the U.S. These and later reading
tests typically measure comprehension by having students first read a passage
and then answer multiple-choice questions.
The Mc Call-Crabbs reading tests also became important in the development andvalidation of the readability formulas. Later reading tests also used for creating
and testing formulas for adults and children include the Gates-MacGinitie
Reading Tests, the Stanford Diagnostic Reading Test, the California Reading
Achievement Test, the Nelson-Denny Reading Test, the Diagnostic Assessmentof Reading with Trial Teaching Strategies and the National Assessment of
Educational Progress (NAEP).

Grading adult readers
For a long time, no one thought of grading adults, who were considered either
literate or illiterate. This began to change with the first systematic testing ofadults in the U.S. military in 1917. The testing of civilians began in Chicago in
1937.
During that first period, investigators discovered that general readers in the U. S.
were adults of limited reading ability. The average adult was able to read withpleasure nothing but the simplest adult materials, usually cheap fiction orgraphically presented news of the day.
Educators, corporations, and government agencies responded by providing morematerials at different reading levels for adults.

U.S. military literacy surveys—reading on the job
General George Washington first addressed concerns about the reading skills offighters during the Revolutionary War. He directed chaplains at Valley Forge toteach basic skills of reading, writing, and arithmetic to soldiers. Since then, theU.S. armed services has invested more in studying workplace literacy than any
other organization.
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The Principles of Readability

Since the 50s in the U.S., you have to pass a literacy test to join the Armed
Services. From such a test and others, the military learns a lot about youraptitudes, cognitive skills, and ability to perform on the job.
It took a while for the military to develop these tests. Over the years, it changed
the content of the tests and what they measure. Testing literacy advanced in thesegeneral stages:
1.

During World War I, they focused on testing native intelligence.

2.

The military decided that what they were testing was not so much raw
intelligence as reading skills. By World War II, they were focusing onclassifying general learning ability for job placement.

3.

In the 1950s, Congress mandated a literacy requirement for all thearmed services. The resulting Armed Forces Qualification Test
(AFQT) prevented people of the lowest 10% of reading ability fromentering military service. The military then combined AFQT subtest
with other tests, which differed for each service and sorted recruits into
different jobs.

4.

In 1976, with the arrival of the All-Volunteer Force, the military
introduced the Armed Services Vocational Aptitude Battery(ASVAB). All military services used this test battery for both screening
qualified candidates and assessing trainability for classified jobs.

5.

In 1978, an error resulted in the recruitment of more than 200,000
candidates in the lowest 10% category. The military, with the aid of
Congress, decided to keep them. The four military services each createdworkplace literacy programs, with contract and student costs over $70
million. This was a greater enrollment in adult basic education than in
all such programs of 25 states combined. The results of the workplace
literacy programs were considered highly successful, with performanceand promotions “almost normal.”6.

In 1980, the military further launched the largest study ever in job
literacy, the Job Performance Measurement/Enlistment Standards
Project. They invested $36 million in developing measures of job
performance. Over ten years, the project involved more than 15,000
troops from all four military services. Dozens of professionals in
psychological measurement took part in this study.

7.

In 1991, based on these findings, the military raised its standards andcombined the ASVAB with the AFQT and special aptitude tests fromall the services into one battery of 10 tests. Both the Army and Navycontinue to provide workplace-literacy programs for entering recruits
and for upgrading the literacy skills of experienced personnel (Sticht
1995, pp 37-38).

The major findings of the military research were:

Copyright © 2004 William H. DuBay

1.

Measures of literacy correlate closely with measures of intelligence andaptitude.

2.

Measures of literacy correlate closely with the breadth of one’s
knowledge.

3.

Measures of literacy correlate closely to job performance. Hundreds ofmilitary studies found no gap between literacy and job performance.

4.

Workplace literacy programs are highly effective in producing, in abrief period, significant improvements in job-related reading.

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The Principles of Readability

5.

Advanced readers have vast bodies of knowledge and perform wellacross a large set of domains of knowledge. Poor readers perform
poorly across these domains of knowledge. This means that, if
programs of adult literacy are to move students to high levels ofliteracy, they must help them explore and learn across a wide range ofknowledge (Sticht and Armstrong 1994, pp. 37-38).

The military studies indicated that achieving high levels of literacy requires
continued opportunities for life-long learning. Investments in adult literacy
provide a unique and cost-effective strategy for improving the economy, thehome, the community, and the schools.

U.S. civilian literacy surveys
University of Chicago Study Guy Buswell (1937) of the University of Chicago
surveyed 1,000 adults in Chicago with different levels of education. He
measured skills in reading materials such as food ads, telephone directories, andmovie ads. He also used more traditional tests of comprehension of paragraphs
and vocabulary.
Buswell found that reading skills and practices increase as years of education
increase. He suggested that an important role of education is to guide readers toread more, and that reading more leads to greater reading skill. In turn, this maylead one to continue more education, thus leading to greater reading skill.

Fig. 1. Adult literacy in 1937. This study confirmed the relationship between reading
skill and years of education completed. Sources: Buswell, G. 1937 pp. 27, 57, 71).

The National Assessment of Educational Progress (NAEP) of 1970-1971
This study tested how students 9, 13, and 17 years old as well adults 26 to 35
years old perform on 21 different tasks. The results showed for the first time how
age affects performance on the same items. This survey showed as children growup, attend school, and become adults, they grow progressively more literate(Sticht and Armstrong, pp. 51-58).
Louis Harris survey of 1970 The Louis Harris polling organization surveyed
adults representing a cross section of the U.S. population. The subjects filled out
five common application forms, including an application for a driver’s license
and a Medicaid application.
The poll was the first of many to show that many U.S. citizens have difficultywith filling out forms. The Medicaid form was difficult, with only 54 percent of
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The Principles of Readability

those with an 8th grade education or less getting 90-100 percent correct. Even
many college-educated adults had trouble completing the Medicaid form (Sticht
and Armstrong, pp. 59-62).
Adult Functional Reading Study of 1973 This study used householdinterviews to find out the literacy practices of adults. It used a second household
sample to assess literacy skills.
Over all 170 items used in the study, over 70 percent of the respondents scored
70 percent correct or better. As a trend, adults with more education performed
better on the test than those with less.
As with Buswell's study, both literacy skills and literacy practices correlated
closely with education. Book and magazine reading correlated more closely withyears of education than did newspaper reading. Altogether, the adults reported
that they spent about 90 minutes a day in reading materials such as forms, labels,
signs, bills, and mail. (Sticht and Armstrong, pp. 63-66).
Adult Performance Level Study of 1971 This study began as a project fundedby the U. S. Office of Education. It introduced "competency-based" education,
directing adult education to focus on achieving measurable outcomes. By 1977,
two-thirds of the states had set up some form of "competency-based" adult basic
education.
The test included over 40 common and practical tasks, such as filling out acheck, reading the want ads, addressing an envelope, comparing advertised
products, filling out items on a 1040 tax form, reading a tax table, and filling out
a Social Security application. Results showed the high correlation between
performance on all tasks and literacy (Sticht and Armstrong, pp. 67-98).
What a Reading Grade Level Means
The reading grade level of a text depends on the use of the text. If the text isused for independent, unassisted, or recreational use, the reading grade level
will be higher than a text destined for classroom use and optimum learning
gain. In other words, the same text will be easier for those with more advanced
reading skills (with a higher grade level) and harder for those with less (and witha lower grade level). See the “Problem of Optimal Difficulty” below.
The grade of completed education is no indication of one’s reading level.
thAverage high-school graduates read at the 9 -grade level, which means a large
number reads below that level. Those who pursue special domains ofknowledge may develop higher levels of reading skill in those specialties thanthey have for general reading. Thus, college graduates, who prefer to readth
general materials at the 10 -grade level, may prefer more difficult texts within
their own specialty. Students who are poor readers of general classroom
material are often able to master difficult treatments of subjects that appeal tothem.

Young Adult Literacy Survey of 1985 This study of young adults (17-25) andthe adult study that followed in 1992 both measured the literacy the same way inthree areas:•

Prose literacy—meaning of selected texts•

Document literacy—finding information on a form such as a bus
schedule.

•

Quantitive literacy—mathematical and spatial tasks.

Both studies used a literacy scoring range of 1 to 500 and the five levels of skill
defined by the National Assessment of Educational Progress (1985). John
Carroll (1987) estimated the corresponding reading-grade levels as shown in
Table 1.
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The Principles of Readability

NAEP Level

Literacy Score

Grade Level

I

Rudimentary

150

1.5

II

Basic

200

3.6

III Intermediate

250

7.2

IV Adept

300

12

V

350

16+

Advanced

Table 1. NAEP proficiency levels and the reading-grade-level
equivalents.

The young adult survey by the NAEP (1985) found that only 40 percent ofyoung adults 17 to 25 no longer in high school, and 17 years old and in highschool, read at a 12th-grade level. Large numbers leave high school still readingat the 8th-grade level or lower. The 1990 census showed that 24.8 percent of
adults did not graduate from high school.
The National Adult Literacy Survey (NALS) of 1992 This U.S. Government
study sampled 26,000 adults, representing 191 million adults. In 1993, it
published the first of a number of reports on this survey entitled, "Adult Literacy
in America” (National Center for Education Statistics 1993, 1999, 2001).
This study used the same tests as the Young Adult Literacy Survey and reported
data with the same five levels of skill.
Literacy Skill

Level 1

Level 2

Level 3

Level 4

Level 5

Prose

21%

27%

32%

17%

3%

Document

23%

28%

31%

15%

3%

Quantitative

22%

25%

31%

17%

4%

Table 2. Percentages of adults in the U.S. in each of the five NAEP skill levels for each
literacy skill (Sticht and Armstrong 1995, p. 113).

The data in this table suggest 40 to 44 million adults in the U.S. are in Level 1,
defined as “functionally illiterate, not having enough reading skills for daily
life.” Some 50 million are in Level 2. This means the percentage of adults who
struggle at Levels 1 and 2 (below the 5th-grade level) in the U.S. reaches 48
percent.
The report confirmed that numeracy (quantitative) skills increase with reading
skills. Adults of different reading skills not only have different worldviews but
also different life experiences. Forty-three percent of adults with low-literacy
skills live in poverty, 17% receive food stamps, and 70% have no job or parttime job. Over 60 % of frontline workers producing goods have difficulty
applying information from a text to a task. More than 20% of adults read below
the sixth-grade level, far below the level needed to earn a living wage.
Adults at Level 1 earned a median income of $240 a week, while those at Level
5 earned $681. Seventy percent of prisoners are in the lowest two levels.
In support of these figures, the number of companies reporting shortages ofskilled workers doubled between 1995 and 1998. Ninety percent of Fortune
1000 executives reported that low literacy is hurting productivity andprofitability. In one survey, more than half of the responding company
representatives said that high school graduates applying for jobs are not literateenough to hire.
Low levels of literacy have caused costly and dangerous mistakes in theworkplace. There are other costs in billions of dollars in the workplace resulting
from low productivity, poor quality of products and services, mistakes,
absenteeism, and lost management time.
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The Principles of Readability

The Adult Literacy Survey also confirmed the effects of literacy on health care.
Since 1974, when health officials became aware of the effects of low literacy onhealth, literacy problems have grown. A more complex health-care system
requires better reading skills to negotiate the system and take more responsibility
for self-care.
Using a nationally representative sample of the U.S. adult population age 16 andolder, the National Academy (2002) on an Aging Society examined the impactof literacy on the use of health care services. The study found that people withlow health-literacy skills use more health care services.
Among adults who stayed overnight in a hospital in 1994, those with low healthliteracy skills averaged 6 percent more hospital visits, and stayed in the hospitalnearly 2 days longer than adults with higher health literacy skills. The added
health-care costs of low literacy are estimated at $73 billion in 1998 dollars. This
includes $30 billion for the Level 2 population plus $43 billion for the Level 1
population. The total is about what Medicare pays for doctor services, dental
services, home health care, prescription drugs, and nursing-home care combined.
Low literacy is not chiefly the problem of immigrants, the elderly, high school
dropouts, or people whose first language is not English. Low literacy is aproblem that knows no age, education, income levels, or national origins. Most
people with low literacy skills were born in this country and have English as
their first language.
One solution to the problem of low literacy of adults is more government andcorporate support for adult literacy programs. Workplace literacy programs havecost-effective and lasting results. Another solution is to produce more texts thatare written for people of diverse reading skills.

Challenges for technical communicators
The lessons of the literacy studies for technical communicators are obvious:•

Low and intermediate literacy skills are a big problem for large
numbers of users of technical documents. Providing technicaldocuments at their levels will advance both their technical and reading
skills.

•The larger the audience, the more it will include the average reading
habits and skills of the public as determined by the literacy surveys.

•

The more critical the information is for safety and health, the greater isthe need for increased readability.

The finding that the great majority of adult readers are mid-range, intermediate
readers brings to us in technical communication new opportunities andchallenges.
Intermediate readers represent a large audience that technical documents havebeen missing. Go into any library or bookstore, and you will find few technicalor scientific publications in the “Young Adult” section, or elsewhere written atthe 7th to 9th-grade level. On the Internet, there is the same scarcity ofintermediate technical materials.
For example, a small sampling of the author’s shows that the support sections of
the Apple and Microsoft Web sites are written at advanced level of 10th grade
and up. The technical books for Dummies and Idiots, while written in a casualstyle, are often at the 10th-grade level and up. Like the car-safety seat
instructions, these technical documents are too difficult for 80 percent of adultreaders in the U.S. Ironically, the user manual that comes with the CorelDraw

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The Principles of Readability
program is written at the 7th-grade level, making it fit for a much larger audience
than its Dummies counterpart.
Considering the keen interest that intermediate readers of all ages can have intechnical matters, this literacy gap is troubling. While some highly motivatedreaders are able to master difficult technical materials, we cannot assume thateveryone will do so. To the contrary, the difficulty of technical materials hastaught many if not most readers of intermediate skill not to look for technicalhelp in written texts. Helpful text means not only providing readers accurate
information but also information written at the reading levels they need.

The Classic Readability Studies
The first aim of the classic readability studies was to develop practical methods
to match reading materials with the abilities of students and adults. These efforts
centered on making easily applied readability formulas which teachers andlibrarians could use.
The first adult literacy surveys in the U.S. in the 1930s brought new concerns
about providing graded texts for adults. For the rest of the century, publishers,
librarians, teachers, and investigators addressed that need with new methods ofdetermining the reading level of texts.
The classic readability studies include these landmark issues:•

L. A. Sherman and the statistical analysis of literature.

•

The vocabulary-frequency lists•

The classic readability formulas

L. A. Sherman and the statistical analysis of literature
Down through the centuries, many had written about the differences between an“ornate” and “plain” style in English.
In 1880, a professor of English Literature at the University of Nebraska, Lucius
Adelno Sherman, began to teach literature from a historical and statistical pointof view.
He compared the older prose writers with more popular modern writers such as
Macaulay (The History of England) and Ralph Waldo Emerson. He noticed aprogressive shortening of sentences over time.
He decided to look at this statistically and began by counting average sentence
length per 100 periods. In his book (1893), Analytics of Literature, A Manual for
the Objective Study of English Prose and Poetry, he showed how sentencelength averages shortened over time:•

Pre-Elizabethan times: 50 words per sentence•

Elizabethan times: 45 words per sentence•

Victorian times: 29 words per sentence•

Sherman’s time: 23 words per sentence.

In our time, the average is down to 20 words per sentence.
Sherman’s work set the agenda for a century of research in reading. It proposed
the following:

Copyright © 2004 William H. DuBay•

Literature is a subject for statistical analysis.

•

Shorter sentences and concrete terms increase readability.

•

Spoken language is more efficient than written language.

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The Principles of Readability•

Over time, written language becomes more efficient by becoming more
like spoken language.

Sherman also showed how individual writers are remarkably consistent in theiraverage sentence lengths. This consistency was to become the basis for thevalidity of using samples of a text rather than the whole thing for readability
prediction.
Sherman was the first to use statistical analysis for the task of analyzing
readability, introducing a new and objective method of literary criticism.
Another of Sherman’s discoveries was that over time sentences not only became
shorter but also simpler and less abstract. He believed this process was due to the
influence of the spoken language on written English. He wrote (p. 312):
Literary English, in short, will follow the forms of the standard spoken
English from which it comes. No man should talk worse than he writes,
no man writes better than he should talk…. The oral sentence is clearest
because it is the product of millions of daily efforts to be clear andstrong. It represents the work of the race for thousands of years in
perfecting an effective instrument of communication.
Linguistic research later confirmed Sherman’s view of the relationship between
spoken and written language. Rudolf Flesch (1946) wrote that English isfollowing written Chinese in making language simpler by substituting standard
word order (subject-verb-object) for more complex grammar.
According to Flesch, Chinese is “the most grown-up talk of mankind. It is theway people speak who started to simplify their language thousands of years
ago and have kept at it ever since….
(p. 12).
“Among the world’s great languages, therunner-up to Chinese is English. It’s
simpler, more flexible, more practical thanany other Western language because it hasgone furthest in losing inflections andstraightening out irregularities” (p. 20)
Sherman’s most important point was theneed to involve the reader. He wrote:

Fig.2. In Analytics of Literature, L.A.
Sherman looked at literature statistically.
He showed the importance of averagesentence length and the relationshipbetween spoken and written English.

The universally best style is not a thingof form merely, but must regard the
expectations of the reader as to the spirit
and occasion of what is written. It is notaddressed to the learned, but to allminds. Avoiding book-words, it will use
only the standard terms and expressionsof common life… It will not run in longand involved sentences that cannot
readily be understood. Correct in all
respects, it will not be stiff; familiar, butsafely beyond all associations ofvulgarity (p. 327).

Vocabulary-Frequency Lists
During the 1920s, two major trends stimulated a new interest in readability:
1.

Copyright © 2004 William H. DuBay

A changing school population, especially an increase in “first
generation” secondary school students, the children of immigrants.
Teachers reported that these students found textbooks too difficult.

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The Principles of Readability

2.

The growing use of scientific tools for studying and objectively
measuring educational problems.

One such tool, Thorndike’s Teacher’s Word Book (1921), was the first extensive
listing of words in English by frequency. It provided teachers with an objectivemeans for measuring the difficulty of words and texts. It laid the foundation foralmost all the research on readability that would follow.
Its author, psychologist Edward. L. Thorndike of Columbia University, noticed
that teachers of languages in Germany and Russia were using word counts tomatch texts with students. The more frequent a word is used, they found, themore familiar it is and the easier to use. As we learn and grow, our vocabulary
grows as does our ability to master longer and more complex sentences. How
much that continues to grow depends on how much reading is done throughoutlife.
A vocabulary test on the meaning of words is the strongest predictor of verbal
and abstract intellectual development. The knowledge of words has always beena strong measure of a reader’s development, reading comprehension, and verbal
intelligence. Chall and Dale (1995, p. 84) write, “It is no accident thatvocabulary is also a strong predictor of text difficulty.”
It happens that the first words we learn are the simplest and shortest. These first,
easy words are also the words we use most frequently. Most people do notrealize the extent of this frequency. Twenty-five percent of the 67,200 words
used in the 24 life stories written by university freshmen consisted of these tenwords: the, I, and, to, was, my, in, of, a, and it (Johnson, 1946). The first 100
most frequent words make up almost half of all written material. The first 300
words make up about 65 percent of it (Fry et al, 1993).
Around 1911, Thorndike began to count the frequency of words in English texts.
In 1921, he published The Teacher’s Word Book, which listed 10,000 words byfrequency of use. In 1932, he followed up with A Teacher’s Word Book of
20,000 Words, and in 1944 with Irving Lorge, A Teacher’s Word Book of 30,000
Words.
Until computers came along, educators, publishers, and teachers commonly used
word-frequency lists to evaluate reading materials for their classes. Thorndike’s
work also was the basis for the first readability formulas for children’s books.
After Thorndike, there was extensive research on vocabulary. The high markcame in Human Behavior and The Principle of Least Effort by Harvard’s
George Kingsley Zipf (1949).
Zipf used a statistical analysis of language to show how the principle of least
effort works in human speech. Zipf showed that, in many languages, there is amathematical relationship between the hard and easy words, now called Zipf’s
curve. This notion of saving energy is a central feature of language and is one ofthe principle bases of research on the frequency of words.
Klare (1968), reviewing the research on word frequency, concludes: “Not only
do humans tend to used some words much more often than others, they recognize
more frequent words more rapidly than less frequent, prefer them, andunderstand and learn them more readily. It is not surprising, therefore, that thisvariable has such a central role in the measurement of readability.”
Dale and O’Rourke: the words Americans know In 1981, publishers of theWorld Book Encyclopedia published The Living Word Vocabulary: A National
Vocabulary Inventory by Edgar Dale and Joseph O’Rourke. The authors based
this work on the earlier work of Thorndike and others as well as on a 25-year
study of their own. It contained the grade-level scores of the familiarity of
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The Principles of Readability

44,000 words. For the first time, it gave scores for each of the meanings a word
can have and the percentage of readers in the specified grade who are familiar
with the word.
The authors obtained the familiarity scores by giving a three-choice test to
students from the 4th to the 16th grade in schools and colleges throughout the
U.S. The editors of the encyclopedia also used the scores to test the readabilityof the articles they published. Field tests of the encyclopedia later confirmed thevalidity of the word scores. This work is exceptional in every respect and isconsidered by many to be the best aid in writing for a targeted grade level.

Fig. 3. Sample entries from The Living Word Vocabulary. This
work featured not only grade level and a short definition, butalso the percentage of readers in that grade who know the
word. The editors of World Book Encyclopedia used thisinformation as one of the reading-level tests for their entries(Dale and O’Rourke 1981).
In the preface, the Editorial Director of the encyclopedia W. H. Nault wrote (p.
v) that this work marked “the beginning of a revolutionary approach to the
preparation and presentation of materials that fit not only the reading abilities,
but the experience and background of the reader as well.”
Although this work is out of print, you can find it at libraries and used bookshops
along with other graded vocabularies and word-frequency lists such as The
American Heritage Word Frequency Book.

The Classic Readability Formulas
Harry D. Kitson—Different readers, different styles Psychologist Harry D.
Kitson (1921) published The Mind of the Buyer, in which he showed how andwhy readers of different magazines and newspapers differed from one another.
Although he was not aware of Sherman’s work, he found that sentence length
and word length measured in syllables are important measures of readability.
Rudolph Flesch would incorporate both these variables in his Reading Ease
formula 30 years later.
Although Kitson did not create a readability formula, he showed how his
principles worked in analyzing two newspapers, the Chicago Evening Post andthe Chicago American and two magazines, the Century and the American. He
analyzed 5000 consecutive words and 8000 consecutive sentences in the four
publications. His study showed that the average word and sentence length wereshorter in the Chicago American newspaper than in the Post, and the American
magazine’s style simpler than the Century’s, accounting for the differences intheir readership.
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The Principles of Readability

The first readability formula Bertha Lively and S. L. Pressey (1923) wereconcerned with the practical problem of selecting science textbooks for juniorhigh school. The books were so overlaid with technical words that teachers spent
all class time teaching vocabulary. They argued that it would be helpful to have a
way to measure and reduce the “vocabulary burden” of textbooks.
Their article featured the first children’s readability formula. It measured thenumber of different words in each 1,000 words and the number of words not on
the Thorndike list of 10,000 words. Their method produced a correlation
coefficient of .80 when tested on 700 books.
In reading research, investigators look for correlations instead of causes. A
correlation coefficient (r = ) is a descriptive statistic that can go from +1.00 to
0.0 or from 0.0 to –1.00. Both +1.00 and –1.00 represent a perfect correlation,
depending on whether the elements are positively or negatively correlated.
A coefficient of 1.00 shows that, as one element changes, the other elementchanges in the same (+) or opposite (-) direction by a corresponding amount. A
coefficient of .00 means no correlation, that is, no corresponding relationshipthrough a series of changes.
For example, if a formula should predict a 9th-grade level of difficulty on a 7thgrade text, and, if at all grade levels, the error is in the same direction and by acorresponding amount, the correlation could be +1.00 or at least quite high. If,
on the other hand, a formula predicts a 9th-grade level for a 6th-grade text, an 8th
grade level for a 10th-grade text, and has similar variability in both directions, thecorrelation would be very low, or even 0.00.
Squaring the correlation coefficient ( r2 = ) gives the percentage ofaccountability for the variance. For example, the Lively and Pressey formula
above accounts for 64% (.802) of the variance of the text difficulty.
Other early school formulas Mabel Vogel and Carleton Washburne (1928) of
Winnetka, Illinois carried out one of the most important studies of readability.
They were the first to study the structural characteristics of the text and the firstto use a criterion based on an empirical evaluation of text. They studied ten
different factors including kinds of sentences and prepositional phrases, as well
as word difficulty and sentence length. Since, however, many factors correlated
highly with one another, they chose four for their new formula.
Following Lively and Pressey, they validated their formula, called the Winnetka
formula, against 700 books that had been named by at least 25 out of almost
37,000 children as ones they had read and liked. They also had the mean readingscores of the children, which they used as a difficulty measure in developing
their formula. Their new formula correlated highly ( r = .845) with the reading
test scores.
With this formula, investigators knew that they could objectively match thegrade level of a text with the reading ability of the reader. The match was not
perfect, but it was better than subjective judgments. The Winnetka formula, thefirst one to predict difficulty by grade levels, became the prototype of modern
readability formulas.
Vogel and Washburne’s work stimulated the interest of Alfred S. Lewerenz(1929, 1929a, 1935, 1939), who produced several new readability formulas for
the Los Angeles School District.
W. W. Patty and W. I. Painter (1931) discovered the year of highest burden inhigh school is the sophomore year. They also developed a formula to measure
the relative difficulty of textbooks based on a combination of frequency as

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The Principles of Readability

determined by the Thorndike list and vocabulary diversity (the number ofdifferent words in a text).
With the rise of the plain-language movement in the 1960s, several critics of theformulas claimed that the formulas do not test comprehensibility (Kern 1979,
Duffy and Kabance 1981, Duffy 1985). The research, however, shows that fromthe beginning their scores correlate well with comprehension difficulty as
measured by reading tests. The formulas rate very well when compared withother widely used psychometric measurements such as reading tests (Chall andDale 1995). Their validity correlations make them useful for predicting thecomprehension difficulty of texts (Bormuth 1966).
Waples and Tyler: What adults read During the Depression in the ‘30s, adulteducation and the increased use of libraries stimulated studies in reading.
Sociologists studied “who reads what and why over consecutive periods,”
looking at reading as an aspect of mass communication.
Douglas Waples and Ralph W. Tyler (1931) published What People Want toRead About, a comprehensive, two-year study of adult reading interests. Insteadof using the traditional library circulation records to determine reading patterns,
they interviewed people divided by sex and occupation into 107 different groups.
It showed the types and styles of materials that people not only read but also
want to read. It also studied what they did not read and why.
They found that the reading of many people is limited because of the lack ofsuitable material. Readers often like to expand their knowledge, but the readingmaterials in which they are interested are too difficult.
Ralph Ojemann: The difficulty of adult materials The year 1934 marked the
beginning of more rigorous standards for the formulas. Ralph Ojemann (1934)
did not invent a formula, but he did invent a method of assessing the difficulty ofmaterials for adult parent-education materials. His criterion was 16 passages ofabout 500 words taken from magazines. He was the first to use adults toestablish the difficulty of his criterion. He assigned each passage the grade levelof adult readers who were able to answer at least one-half of the multiple-choicequestions about the passage.
Ojemann was then able to correlate six factors of vocabulary difficulty and eightfactors of composition and sentence structure with the difficulty of the criterion
passages. He found that the best vocabulary factor was the difficulty of words as
stated in the Thorndike word list.
Even more important was the emphasis that Ojemann put on the qualitative
factors such as abstractness. He recommended using his 16 passages forcomparing and judging the difficulty of other texts, a method that is now known
as scaling (See “Text leveling” below). Although he was not able to express thequalitative variables in numeric terms, he succeeded in proving they could not beignored.
Dale and Tyler: Adults of limited reading ability After working with Waples,
Ralph Tyler became interested in adults of limited reading ability. He joinedwith Edgar Dale to publish (1934) their own readability formula and the firststudy on adult readability formulas. The specific contribution of this study was
the use of materials specifically designed for adults of limited reading ability.
Their criterion for developing the formula was 74 selections on personal healthtaken from magazines, newspapers, textbooks, and adaptations from children’s
health textbooks. They determined the difficulty of the passages with multiplechoice questions based on the texts given to adults of limited reading ability.

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The Principles of Readability

From the 29 factors that had been found significant for children’scomprehension, they found ten that were significant for adults. They found thatthree of these factors correlated so highly with the other factors that they alone
gave almost the same prediction as the combined ten. They were:•

Number of different technical words.

•

Number of different hard non-technical words.

•

Number of indeterminate clauses.

They combined these three factors into a formula to predict the proportion of
adult readers of limited reading ability who would be able to understand thematerial. The formula correlated .511 with difficulty as measured by multiplechoice reading tests based on the 74 criterion selections.
The Ojemann and Dale-Tyler studies mark the beginning of work on adultformulas that would continue unabated until the present time.
Lyman Bryson: Books for the average reader During the depression of the
1930’s, the government in the U.S. put enormous resources into adult education.
Bryson Lyman first became interested in non-fiction materials written for the
average adult reader while serving as a leader in adult-education meetings in
New York City. What he found was that what kept people from reading more
was not lack of intelligence, but the lack of reading skills, a direct result oflimited schooling.
He also found out there is a tendency to judge adults by the education theirchildren receive and to assume the great bulk of people have been through highschool. At that time, 40 to 50 million people had a 7th to 9th grade education andreading ability.
Writers had assumed that readers had an equal education to their own or at leastan equal reading ability. Highly educated people failed to realize just how mucheasier it is for them to read than it is for an average person. They found it
difficult to recognize difficult writing because they read so well themselves.
Although college and business courses had long promoted ideas expressed in a
direct and lucid style, Bryson found that simple and clear language was rare. He
said such language results from “a discipline and artistry which few people whohave ideas will take the trouble to achieve… If simple writing were easy, manyof our problems would have been solved long ago” (Klare and Buck, p. 58).
Bryson helped set up the Readability Laboratory of the Columbia University
Teachers College with Charles Beard and M. A. Cartwright. The purpose of thelaboratory was not to rewrite the classics or to help the beginning reader. The
purpose was to produce readable books on serious subjects for the average
citizen.
Bryson understood that people with enough motivation and time could readdifficult material and improve their reading ability. Experience, however,showed him that most people do not do that.
Perhaps Bryson’s greatest contribution was the influence he had on his two
students, Irving Lorge and Rudolf Flesch.
Gray and Leary: what makes a book readable William S. Gray and Bernice
Leary (1935) published a landmark work in reading research, What Makes aBook Readable. Like Dale and Tyler’s work, it attempted to discover whatmakes a book readable for adults of limited reading ability.
Their criterion included 48 selections of about 100 words each, half of themfiction, taken from the books, magazines, and newspapers most widely read by
adults. They established the difficulty of these selections by a readingCopyright © 2004 William H. DuBay

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The Principles of Readability

comprehension test given to about 800 adults designed to test their ability to get
the main idea of the passage.
No subsequent work has examined readability so thoroughly or investigated somany style elements or the relationships between them. The authors first
identified 228 elements that affect readability and grouped them under these four
headings:
1.

Content

2.

Style

3.

Format4.

Features of Organization

The authors found that content, with a slight margin over style, was mostimportant. Third in importance was format, and almost equal to it, “features oforganization,” referring to the chapters, sections, headings, and paragraphs thatshow the organization of ideas (See Figure 4).

Fig 4. The four major categories of readability (Gray and Leary, p. 31).

They found they could not measure content, format, or organization statistically,
though many would later try (See below, “The measurement of content”). While
not ignoring the other three causes, Gray and Leary concentrated on 80 variablesof style, 64 of which they could reliably count. They gave several tests to about athousand people. Each test included several passages and questions to show how
well the subjects understood them.

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The Principles of Readability

Fig. 5. The four basic elements of reading ease.

Having a measure, now, of the difficulty of each passage, they were able to see
what style variables changed as the passage got harder. They used correlation
coefficients to show those relationship.
Of the 64 countable variables related to reading difficulty, those withcorrelations of .35 or above were the following (p.115):
1.

Average sentence length in words: -.52 (a negative correlation, that is,
the longer the sentence the more difficult it is).

2.

Percentage of easy words: .52 (the larger the number of easy words theeasier the material).

3.

Number of words not known to 90% of sixth-grade students: -.51

4.

Number of “easy” words: .51

5.

Number of different “hard” words: -.50

6.

Minimum syllabic sentence length: -.49

7.

Number of explicit sentences: .48

8.

Number of first, second, and third-person pronouns: .48

9.

Maximum syllabic sentence length, -.47

10. Average sentence length in syllables, -.47
11. Percentage of monosyllables: .43
12. Number of sentences per paragraph: .43
13. Percentage of different words not known to 90% of sixth-gradestudents: -.40
14. Number of simple sentences: .39
15. Percentage of different words: -.38
16. Percentage of polysyllables: -.38
17. Number of prepositional phrases: -35
Although none of the variables studied had a higher correlation than .52, theauthors knew by combining variables, they could reach higher levels ofcorrelation. Because combining variables that were tightly related to each other
did not raise the correlation coefficient, they needed to find which elements werehighly predictive but not related to each other.
Gray and Leary used five of the above variables, numbers 1, 5, 8, 15, and 17, tocreate a formula, which has a correlation of .645 with reading-difficulty scores.
An important characteristic of readability formulas is that one that uses morevariables may be only minutely more accurate but much more difficult tomeasure and apply. Later formulas that use fewer variables may have higher
correlations.
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The Principles of Readability

Gray and Leary’s work stimulated an enormous effort to find the perfectformula, using different combinations of the style variables. In 1954, Klare andBuck listed 25 formulas for children and another 14 for adult readers. By 1981,
Klare noted there were over 200 published formulas.
Research eventually established that the two variables commonly used inreadability formulas–a semantic (meaning) measure such as difficulty ofvocabulary and a syntactic (sentence structure) measure such as average sentence
length–are the best predictors of textual difficulty.
Some experts consider the number of morphemes for each 100 words to be amajor contributor to semantic (meaning) difficulty and the number of Yngve
word depths (branches) in each sentence to be a major contributor to syntactic(sentence) difficulty. One study (Coleman 1971) showed that Flesch’s index ofsyllables for each 100 words correlates .95 with morpheme counts. Anotherstudy (Bormuth 1966) found that the number of words in each sentence
correlates .86 with counts of Yngve word depths. Measuring the average numberof syllables per word and the number of words in each sentence is a much easier
method and almost as accurate as measuring morphemes and word depths.
Formula limitations Readability researchers have long taken pains torecommend that, because of their limitations, formulas are best used inconjunction with other methods of grading and writing texts. Ojemann (1934)
warned that the formulas are not to be applied mechanically, a caution expressed
throughout readability literature. Other investigators concerned with thedifficulty and density of concepts were Morriss and Holversen (1938) and Dolch(1939). E. Horn (1937) warned against the mechanical use of the word lists in
the re-writing of books for social studies.
George Klare and colleagues (1969) stated, “For these reasons, formula scores
are better thought of as rough guides than as highly accurate values. Used as
rough guides, however, scores derived from readability formulas provide quick,easy help in the analysis and placement of educational material.”
Readability researchers such as Flesch (1949, 1964, 1979), Klare and Buck
(1954), Klare (1980), Gunning (1952), Dale (1967), Gilliland (1972), and Fry(1988) wrote extensively on the other rhetorical factors that require attention
such as organization, content, coherence, and design. Using the formulas
creatively along with techniques of good writing results in greater
comprehension by an audience of a specified reading ability (Klare 1976, Challand Conard 1991).
Irving Lorge: Consolidating the research. Irving Lorge (1938) published The
Semantic Count of the 570 Commonest English Words, a frequency count of themeaning of words rather than the words themselves. He later he was co-authorof E. L. Thorndike’s last book, The Teacher’s Word Book of 30,000 Words(1944).
Lorge was interested in psychological studies of language and human learning.
At Columbia, he came under the influence of Bryson at the Readability Lab. He
wanted a simple formula for predicting the difficulty of children’s books in termsof grade scores. For the criterion for his own formula, Lorge (1939) used the 376
selections taken from the McCall-Crabbs Standard Test Lessons in Reading(1926). He then standardized those passages on the basis of questions answeredby children in terms of the Thorndike-McCall Reading Scale. Using correlation
techniques, he was able to show that various combinations of factors gave
predictions of higher accuracy than the Gray-Leary formula.

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The Principles of Readability

Though created for children’s reading, Lorge’s formula was soon widely used
for adult material as well. Where Gray and Leary’s formula had five elements,
Lorge’s had three, setting a trend that was to follow:•

Average sentence length in words•

Number of prepositional phrases per 100 words•

Number of hard words not on the Dale list of 769 words.

Lorge’s use of the McCall-Crabbs Standard Test Lessons in Reading as acriterion of difficulty greatly simplified the problem of matching readers to texts(Klare, 1985). Although these passages were far from ideal, they had fewer faults
than other passages used as criteria. They remained the standard for readability
studies until the Bormuth studies in 1969.
The problem of communication to masses of people became especially obvious
during World War II. The government bureaus and the armed services needed
efficient ways of assessing the readability of their materials. Lorge’s formula was
one of the first, and it came into wide use (Klare and Buck, p. 59).
Rudolf Flesch and the art of plain writing The one perhaps most responsible
for publicizing the need for readability was Rudolf Flesch, a colleague of Lorge
at Columbia University. Besides working as a readability consultant, lecturer,
and teacher of writing, he published a number of studies and nearly 20 popular
books on English usage and readability. His best-selling books included The Art
of Plain Talk (1946), The Art of Readable Writing (1949), The Art of Clear
Thinking (1951), Why Johnny Can’t Read —And What You Can Do About It(1955), The ABC of Style: A Guide to Plain English (1964), How to Write in
Plain English: A Book for Lawyers and Consumers (1979).
Flesch was born in Austria and got a degree in law from the University of
Vienna in 1933. He practiced law until 1938, when he came to the U.S. as a
refugee from the Nazis.
Since his law degree was not recognized, he worked
several other jobs, one of them in the shipping
department of a New York book manufacturer.
In 1939, he received a refugee’s scholarship atColumbia University. In 1940, he received abachelor’s degree with honors in library science. Thatsame year, he became an assistant to Lyman Bryson in
the Teachers’ College Readability Lab.
In 1942, Flesch received a master’s degree in adulteducation. The next year, he received a Ph.D. ineducational research for his dissertation, “Marks of a
Readable Style” (1943). This paper set a course for his
career and that of readability.
Fig. 6. Rudolf Flesch. The firstedition of The Art of Plain Talk in1946 was a best seller. The
readability formulas it featured
started a revolution in journalism
and business communication.

In his dissertation, Flesch published his firstreadability formula for measuring adult readingmaterial. One of the variables it used was affixes andanother was “personal references” such as personalpronouns and names. Publishers quickly discoveredthat Flesch’s formula could increase readership by 40
to 60 percent. Investigators in many fields ofcommunication began using it in their studies.

In 1948, Flesch published a second formula with two parts. The first part, theReading Ease formula, dropped the use of affixes and used only two variables,
the number of syllables and the number of sentences for each 100-word sample.
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The Principles of Readability

It predicts reading ease on a scale from 1 to 100, with 30 being “very difficult”
and 70 being “easy.” Flesch (p. 225) wrote that a score of 100 indicates reading
matter understood by readers who have completed the fourth grade and are, inthe language of the U.S. Census barely “functionally literate.”
The second part of Flesch’s formula predicts human interest by counting thenumber of personal words (such as pronouns and names) and personal sentences(such as quotes, exclamations, and incomplete sentences).
The formula for the updated Flesch Reading Ease score is:Score = 206.835 – (1.015 x ASL) – (84.6 x ASW)
Where:
Score = position on a scale of 0 (difficult) to 100 (easy), with 30 = very
difficult and 70 = suitable for adult audiences.
ASL = average sentence length (the number of words divided by the numberof sentences).
ASW = average number of syllables per word (the number of syllables
divided by the number of words).
This formula correlates .70 with the 1925 McCall-Crabbs reading tests and .64
with the 1950 version of the same tests.
In The Art of Readable Writing, Flesch (1949, p. 149), described his Reading
Ease scale in this way:
Reading
Ease Score

Style
Description

Estimated Reading
Grade

Estimated Percent
of U.S. Adults(1949)

0 to 30:
30 to 40:

Very Difficult

College graduate

4.5

Difficult13th to 16th grade

33

th

th

50 to 60:

Fairly Difficult

10 to 12 grade

54

60 to 70:

Standard

8th and 9th grade

83

70 to 80:

Fairly Easy

7th grade88th

80 to 90:Easy

6 grade91

90 to 100:

Very Easy

5th grade

93

Table 3. Flesch’s Reading Ease Scores

Flesch’s Reading Ease formula became the most widely used formula and one of
the most tested and reliable (Chall 1958, Klare 1963).
In an attempt to further simplify the Flesch Reading Ease formula, Farr, Jenkins,
and Paterson (1951) substituted the average number of one-syllable words per
hundred words for Flesh’s syllable count. The modified formula is:
New Reading Ease score = 1.599 nosw – 1.015 sl – 31.517
Where: nosw = number of one-syllable words per 100 words;
sl = average sentence length in words
This formula correlates better than .90 with the original Flesch Reading Ease
formula and .70 with 75% comprehension of 100-word samplings of the McCallCrabbs reading lessons. In 1976, a study commissioned by the U.S. Navy
modified the Reading Ease formula to produce a grade-level score, This popularformula is known as the Flesch-Kincaid formula, the Flesch Grade-Scale formulaor the Kincaid formula (See “The Navy Readability Indexes” below).
In 1949, Flesch published the results of a 10-year study of the editorial content
of several magazines. He found that:•
Copyright © 2004 William H. DuBay

About 45% of the population can read The Saturday Evening Post.

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The Principles of Readability•

Nearly 50% of the population can read McCall’s, Ladies Home
Journal, and Woman’s Home Companion.

•

Slightly over 50% can read American Magazine.

•

80% of the population can read Modern Screen, Photoplay, and threeconfession magazines.

Flesch (1949, pp. 149-150) compared the reading scores of popular magazines
with other variables:
Style

FleschReading
Ease
Score

Average
Sentence
Length in
Words

AverageNo. of Syll.
Per 100
Words

Type ofMagazine

Estimated
SchoolGrade
Completed

Estimated
Percent of
U.S. Adults

Very
Easy

90 to 100

8 or less

123 or less

Comics

4th grade

93

Easy

80 to 90

11

131

Pulp
fiction5th grade91

FairlyEasy

70 to 80

14

139

Slick
fiction6th grade

88Standard

60 to 70

17

147

Digests

7th or 8th
grades83

FairlyDifficult

50 to 60

21

155

QualitySome highschool54

Difficult

30 to 50

25

167

Academic

High schoolor somecollege

33

Very
Difficult

0 to 30

29 or
more

192 ormore

ScientificCollege

4.5

Table 4. Flesch’s1949 analysis of the readability of adult reading materials.

Flesch’s work had an enormous impact on journalism. Like Robert Gunning,
who worked with the United Press, Flesch was a consultant with the AssociatedPress. Together, they helped to bring down the reading grade level of front-pagestories from the 16th to the 11th grade, where they remain today.
The Dale and Chall Original Formula Edgar Dale, for
25 years a professor of education at Ohio State University,
was a respected authority on communications. He worked
his whole life to improve the readability of books,
pamphlets, and newsletters—the stuff of everyday reading.
Dale was one of the first critics of the Thorndike lists. He
claimed it failed to measure the familiarity of words
accurately. He subsequently developed new lists that werelater used in readability formulas.
Fig. 7. Edgar Dale, one of
the creators of The Living
Word Vocabulary,stressed the importanceof vocabulary in assessing
readability.

One of these was a formula he developed with Jeanne
Chall, the founder and director for 20 years of the Harvard
Reading Laboratory. She had led the battle for teaching
early reading systematically with phonics. Her 1967 bookLearning to Read: The Great Debate, brought research tothe forefront of the debate. For many years, she also was
the reading consultant for TV’s Sesame Street and The
Electric Company.

The original Dale-Chall formula (1948) was developed for adults and children
above the 4th grade. They designed it to correct certain shortcomings in the
Flesch Reading Ease formula. It uses a sentence-length variable plus a
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The Principles of Readability

percentage of “hard words”–words not found on the Dale-Chall “long list” of
3,000 easy words, 80 percent of which are known to fourth-grade readers.
To apply the formula:
1.

Select 100-word samples throughout the text (for books, every tenthpage is recommended).

2.

Compute the average sentence length in words.

3.

Compute the percentage of words outside the Dale list of 3,000 words.

4.

Compute this equation:

Score = .1579PDW + .0496ASL + 3.6365
Where:

Raw score = reading grade of a reader who can answer one-half ofthe test questions on a passage.
PDW= Percentage of Difficult Words (words not on the Dale-Challword list)
ASL = Average Sentence Length in words.

The Raw Score needs to be corrected at the higher grades. Dale and Chall
included with their formula the following chart for correcting the Raw Scores.
Raw Score

Dale-Chall Score

4.9 and below

Grade 4 and below

5.0 to 5.9

Grades 5-6

6.0 to 6.9

Grades 7-8

7.0 to 7.9

Grades 9-10

8.0 to 8.9

Grades 11-12

9.0 to 9.9

Grades 13-15 (college)

10 and above

Grades 16 and above (college graduate)

Table 5. Dale-Call score-correction chart.

Of all the formulas produced in the early classic period, validations of thisformula have produced the most consistent, as well as some of the highest
correlations. It correlated .70 with the multiple-choice test scores on the McCallCrabbs reading lessons. You can find a computerized version of this original
formula online at:
http://www.interventioncentral.org/htmdocs/tools/okapi/okapi.shtml
Those interested in manually applying this formula can find the original 1948
Dale-Chall easy word list online at:
http://www.interventioncentral.org/htmdocs/tools/okapi/okapimanual/dalechalllist.shtml

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The Principles of Readability

Robert Gunning and the technique of clear writing
Robert Gunning was a graduate of Ohio State
University. In 1935, he entered the field of textbook
publishing. In the mid-1930s, educators were beginningto see high school graduates who were not able to read.
Gunning realized that much of the reading problem wasa writing problem. He found that newspapers andbusiness were full of “fog” and unnecessary complexity.

Fig. 8. Robert Gunning.
His firm was the first
dedicated to improving
readability of journalism
and business writing.

Gunning was among the first to take the new readability
research into the workplace. In 1944, he founded thefirst consulting firm specializing in readability. During
the next few years, he tested and worked with more than
60 large city daily newspapers and the popularmagazines, helping writers and editors write to theiraudience.

In The Technique of Clear Writing, Gunning (1952) published a readability
formula developed for adults, the Fog Index, which became popular because of
its ease of use. It uses two variables, average sentence length and the number ofwords with more than two syllables for each 100 words.
Grade Level = .4 (average sentence length + hard words)
Where:
Hard words = number of words of more than two syllables
Gunning developed his formula using a 90% correct-score with the McCallCrabbs reading tests. This gives the formula a higher grade criterion than otherformulas except for McLaughlin’s SMOG formula, which is based on a 100%
correct-answer criterion. The grade-level scores predicted by these two formulas
tend to be higher than other formulas.
Gunning found that popular magazines were consistent in their reading levels
over time. He published these correlations between reading levels of different
classes of magazines and their total circulation (p. 35). See Table 6.
Group

Approx. Total
Circulation

Average
Sentence
LengthPercentage
of Hard
Words

Total

Fog
Index

Class

Fewer than 1
million

20

10

30

12

News
Reader’s
DigestAbout 3 million

16

10

26

10

8 million

15

7

22

9

SlicksMore than 10
million

15

5

20

8

Pulps

More than 10
million

15

3

16

6

Table 6. Gunning’s analysis of the readability of adult reading materials.

The validation of the original Fog formula has never been published. According
to this author’s calculations, however, it correlates .93 with the normed reading
texts of Chall et al. (1996), a figure which may account for its popularity.

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The Principles of Readability

Sumner, and Kearl (1958) recalculated the Fog formula using the McCall-Crabbs
reading lessons. The recalculated Fog formula, shown here, correlates .59 withthe reading passages.
Grade level = 3.0680 + .0877 (average sentence length) + .0984 (percentage
of monosyllables)
The publication of the Flesch, Dale-Chall, and Gunning formulas conveniently
marks the end of the first 30 years of classic readability studies. The authors ofthese formulas brought the issue of readability to public attention. They
stimulated new consumer demands for documents in plain language. Finally,
they stimulated new studies, not only on how to improve the formulas, but also
on the other factors affecting reading success.

The New Readability Studies
The new readability was a period of consolidation and deeper study.
Investigators sought to learn more about how the formulas work and how to
improve them.
In the 1950s, several other developments accelerated the study of readability.
The challenges of Sputnik and the demands of new technologies created a needfor higher reading skills in all workers. While the older manufacturing industries
had little demand for advanced readers, new technologies required workers withhigher reading proficiency.
The New Readability studies were characterized by these features:

Copyright © 2004 William H. DuBay•

A community of scholars. The periodical summaries of the progress ofreadability research (Klare 1952, 1963, 1974-75, 1984, Chall 1958,
and Chall and Dale 1995) revealed an exceptionally successful group ofscholars. They were often neglected by the others in the academe
because of their work on practical rather than theoretical problems.
They studied how and why the formulas work, how to improve them,and what they tell us not only about reading, but also about writing.

•

The cloze test. The introduction of the cloze test by Wilson Taylor in1953 opened the way for investigators to test the properties of texts andreaders with more accuracy and detail.

•

Reading ability, prior knowledge, interest, and motivation. A
number of studies looked at the manner in which these reader variables
affect readability.

•

Reading efficiency. While other studies looked at the effects ofreadability on comprehension, these studies looked at the effects onreading speed and persistence.

•

The measurement of content. The influence of cognitive psychology
and linguistics in the 1980s stimulated renewed studies of cognitive andstructural factors in the text and how they can be used to predictreadability.

•

Text leveling. Cognitive and linguistic theory revived interest in thequalitative and subjective assessment of readability. With training,leveling can be effective in assessing the elements of texts notaddressed by the formulas.

•

Producing and transforming text. Several studies examined theeffectiveness of using the formula variables to write and revise texts.
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The Principles of Readability

readability variables can be effective in producing and transforming atext to a required reading level.
•

New readability formulas. Extensive studies of readability by John
Bormuth and others looked at the reliability of a wide range ofmeasurable text variables. They produced an empirical basis for
criterion scores and criterion texts for the development of new formulas
and reworking of old ones.

•

Formula discrepancy A look a the discrepancy between the results ofdifferent formulas and how writers can benefit from it.

A Community of Scholars
Two notable features of readability research were acommunity of scholars and a long research base. The
recognized bibliographer of that effort was George R.
Klare, now Distinguished Professor Emeritus of
Psychology and former Dean of the College of Arts andSciences at the Ohio University. Formerly the Dean of the
Department of Psychology, his field was psychologicalstatistics and testing as well as readability measurement.
Klare not only reviewed readability research (1963, 197475, 1984), but he also directed and participated inlandmark studies and took the results of research to the
public. His reviews established the validity of theformulas and their proper use not only in English, but also
Fig. 9. George Klare. Afterin many other languages. Among Klare’s many important
serving as a navigator for the
publications were:
U.S. Air Force in WWII (inwhich he was shot down andcaptured by the Germans), • Know Your Reader: The Scientific Approach to
Readability, which he wrote with Byron Buck (1954).
Klare became a leading
figure in readability• The Measurement of Readability (1963).
research.
•“Assessing Readability” in the Reading Research Quarterly (1974-75).
The Institute for Scientific Information recognized it as a Citation
Classic, one of the scientific works most frequently cited in otherstudies—with well over 125 citations so far.

•

“A Second Look at the Validity of the Readability Formulas” in The
Journal of Reading Behavior (1976).

•“Readable Technical Writing: Some Observations” in TechnicalCommunication (1977), which won “Best of Show” in the International
Conference of the STC in Dallas in 1978.

•

A Manual for Readable Writing (1975).

•

How to Write Readable English (1980).

•“Readability” in Encyclopedia of Educational Research (1982).

•“Readability” in TheHandbook of Reading Research (1984).

•“Readable Computer Documentation” in the ACM Journal of Computer
Documentation (2000), which covered the latest research in readability.

Critics of the formulas (e.g., Redish and Selzer 1985) complained that thereadability formulas were developed for children and they never were neverformulated or tested with technical documents. The record shows, however, that
popular formulas such as the Flesch Reading Ease and the Kincaid formulas
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The Principles of Readability

were developed mainly for adults and have been tested extensively on adults
with adult materials. For example, Klare (1952) tested the Lorge, Flesch Reading
Ease, and Dale-Chall formulas against the 16 standardized passages of the
Ojemann tests (1934) and the 48 passages of Gray and Leary (1935) tests, alldeveloped for adult readers.
As we will see, several extensive studies (Klare et al. 1955a, Klare et al. 1957,
Klare and Smart 1973, Caylor et al. 1973, Kincaid et al. 1975, Hooke et al.
1979) used materials developed for technical training and regulations in the
military to formulate and test several of today’s most popular formulas such as
the Flesch-Kincaid grade-level formula.
Perhaps Klare's most important studies were those confirming the effects of priorknowledge, reading ability, interest, and motivation on adult reading (Seebelow).

The Cloze Test
Wilson Taylor (1953) of the University of Illinois published “Cloze Procedure:
A New Tool for Measuring Readability.” Taylor cited several difficulties withthe classic readability formulas such as the Flesch and Dale-Chall. He noted, forinstance, that Gertrude Stein’s works measured much easier on the readability
scales than expected.
Taylor argued that words are not the best measure of difficulty but how they
relate to one another. He proposed using deletion tests called cloze tests formeasuring an individual’s understanding of a text. Cloze testing is based on thetheory that readers are better able to fill in the missing words as their reading
skills improve.
A cloze test uses a text with regularly deleted words (usually every fifth word)
and requires the subjects to fill in the blanks. The percentage of words correctly
entered is the cloze score. The lower the score, the more difficult the text.
Because even advanced readers cannot correctly complete more than 65% of thedeleted words correctly in a simple text, texts for assisted reading require a cloze
score of 35% or more. Texts for unassisted reading need a higher score. Cloze
scores line up with scores from multiple-choice tests in the following manner:
PurposeCloze

Multiple-Choice

Unassisted
reading50-60%

70-80%

Instructional,
assisted reading

35-50%

50-60%

Frustration level

Below 35%

Below 50%

Table 7. Comparison of cloze and multiple-choice scores.

For the origins of these scores, see “The Problem of Optimal Difficulty” below.
A cloze test uses a text with selected words deleted and replaced with underlinesof the same length. Having at least 50 blanks in the reading selection increases
the reliability of the test.
To score a cloze test, use the percentage of all the words that are correctly
entered, that is, the right words in the right form (no synonyms), number, person,
tense, voice, and mode. Do not count spelling.
It greatly increases the accuracy of the test to test all the words by using different
versions of the text. If you delete every 5th word, there are five possible versions,each one with a different first deleted word. Divide the subjects into as manygroups as you have versions and give each group a different version.
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The Principles of Readability

Here is a sample cloze test:
The potential for two-way _______ is very strong on ________ Web. As aresult, ________ companies are focused on ________ Web’s marketing
potential. From ________ marketing point of view, ________ virtual worlds
can attract ________curious Web explorers, and ________database engines
can measure ________ track a visitor’s every ________.
See the answers at the end of this article. Note that the standard cloze test does
not provide a list of the correct words to choose from as some online cloze
programs do.
Cloze testing became the object of intensive research, with over a thousandstudies published (Klare 1982). It quickly became popular as a research tool, andtended to complement not the formulas as expected but conventional reading
tests. Unlike multiple-choice tests, cloze tests can provide suggestive information
about individual sentences, clauses, phrases, and words. Cloze tests are suitablefor intermediate and advanced readers. Cloze testing opened the way for much
more intensive studies of the readability formulas, beginning with Bormuth in1966 (see below).

Reading Ability, Prior Knowledge, Interest, and Motivation
The interest factors affecting the readability of children’s literature was taken up
by Gates (1930) and Zeller (1941). One of the interest factors that Gates
mentioned for children was reading ease. Flesch’s early formula for adults(1949) included interest factors for measuring readability. The new researchwould establish that, along with vocabulary and sentence structure, the reader’sreading ability, prior knowledge and motivation are powerful contributors to text
readability.
Prior knowledge and retention A series of studies in the military (Klare et al.
1955a) examined how prior knowledge as well as the text variables affect theretention and the acceptability of technical documents.
The studies were conducted at Sampson Air Force Base in New York andChanute Air Force Base in Illinois using 989 male Air Force enlistees in training
with different versions of the same texts. They used the Flesch Reading Ease,
Dale-Chall, and the Flesch Level-of-Abstraction formulas to rate the texts as
Easy (grade 7), Present (12th grade), and Hard (16th grade).
While simplifying documents and changing the style, they retained all technicalterms and used technical experts to assure that they did not change the content.
This study found the more readable versions resulted in:•Greater and more complete retention.

•

Greater amount read in a given time.

•

Greater acceptability (preference).

The study found that, “…while style difficulty appears to affect immediate
retention of subjects who are naïve regarding material, subjects who haveconsiderable knowledge of the material may profit little if any from an easierstyle of material” (p. 294).
Duffy (1985) criticizes the results of this study. He states that the 8% percent
improvement in comprehension, achieved by dropping the reading level of thetexts eight grades (from the 16th+ grade to the 7th-8th grades—1% improvement
for each grade dropped) is not large enough to justify the effort required.
Duffy underestimates the difficulty of demonstrating the comprehension gainedby changing any textual variable while carefully controlling the other variables
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The Principles of Readability

done in the study. Most researchers are very happy to get any non-chance
improvements in comprehension, the holy grail of reading research.
The difficulty arises from the complexity of reading comprehension and themeans we have of testing it, which are all indirect. Researchers, for example, arenot sure exactly what the results of reading tests are telling them. Do they reveal
comprehension of the text or other artifacts such as prior knowledge, memory, or
the difficulty of the questions?
Studies of the effects of textual variables and writing strategies oncomprehension are very often inconsistent, inconclusive, or non-existent.
Examples include: the use of illustrations (Halbert 1944, Vernon 1946, Omaggio
1979; Felker et al., 1981), schemas (Rumelhart 1984), structural cues
(Spyridakis, 1989, 1989a), highlighting (Klare et al. 1955b, Felker et al.),paragraph length (Markel, et al., 1992), typographic format (Klare 1957), syntax
simplification (Ulijn and Strother 1990), prior knowledge (Richards 1984),
nominalizations, diagrams, parallelism, white space, line graphs, and justified
margins (Felker et al,), “whiz deletions” (Huckin et al. 1991), writer guidelines(McLean 1985), and coherence and cohesion (Freebody and Anderson 1983,
Halliday and Hasan 1976).
No one would say that any of these items are not helpful or do not affectcomprehension. These studies show, however, how difficult it is to detect andmeasure the effect of a reading variable on comprehension. Any significant gain
in comprehension, even a small one, can be important over time and suggests
further study. In this regard, studies confirm that the formulas do very well. See“Producing and transforming texts” below.
Career preferences, aptitudes, and test scores. A further investigation by thesame authors (Klare et al. 1955c) looked into the effect of career aptitude andpreferences on immediate retention. As expected, the subjects with higher degreeof mechanical and clerical aptitude showed consistently higher retention on testscores. There were no significant relationships, however, between career
preferences and retention.
Interest, Prior Knowledge, Readability and Comprehension A study (Klare
1976) of the experiments on the effects of using formulas to revise texts showed
how different levels of motivation and reading ability can skew the results. It
also indicated that the readability of a text is more important when interest is low
than when it is high. The study by Fass and Schumacher (1978) supports thisclaim.
Woern (1977) later showed that prior knowledge and beliefs about the world
affected comprehension significantly. Pearson, Hanson, and Gordon (1979)
discovered significant effects of prior knowledge on the comprehension ofchildren reading about spiders. Spilich, Vesonder, Chiesi, and Voss (1979)
found that subjects having more knowledge about baseball remembered more
information about a baseball episode. Chiesi, Spilich, and Voss (1979) foundthat high-knowledge subjects had better recognition, recall, and anticipation ofgoal outcomes than did low-knowledge subjects.
Entin and Klare (1985) took up the interaction between the readability of the text
and the prior knowledge and interest of the readers. The study used 66 students
enrolled in introductory psychology courses at Ohio University. They were firsttested with the Nelson-Denny Reading Test to determine reading skills. They
were then given a questionnaire on their interest in selected topics and aquestionnaire on their prior knowledge of the terminology used in the test
passages. For test passages, they used 12 selected passages from the World Book
Encyclopedia, six high-interest passages, and six low-interest ones. The passages
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The Principles of Readability
were re-written and normed by judges for content at the 12th and 16th-gradelevels, resulting in 24 passages for the experiment. Then, two cloze tests weremade of each passage, resulting in 48 test passages
This study confirmed that easier readability of a text has more benefits for thoseof less knowledge and interest than those of more. Advanced knowledge of a
subject can “drown out” the effects of an otherwise difficult text.
This study also suggested that when reader interest is high, comprehension is not
improved by writing the material below, rather than at, the grade level of thereaders. When interest is low, however, comprehension is improved by writing
the materials below, rather than at, the reading level of the readers.
Comprehension was improved when the materials are written at the readinglevels of all readers rather than above those levels.

Reading Performance
While early studies used reader comprehension as a measure of readability, newstudies were looking at other performance measures such as:•

Readership•

Reading persistence (or perseverance)•

Reading efficiency

Readability and reader persistence Several studies in the field of journalism
found a significant relationship between reader persistence and readability. Some
used split runs of newspapers to see the effects of improved readability on wide
audiences.
Donald Murphy (1947), the editor of Wallace’s Farmer, used a split run with anarticle written at the 9th-grade level on one run and on at the 6th-grade level on
the other run. He found that increasing readability increased readership up of thearticle 18 percent. In a second test, he took great care not to change anything
except readability, keeping headlines, illustrations, subject matter and theposition the same. He found readership increases of 45% for an article on nylon
and 60% for an article on corn.
Wilbur Schramm (1947) showed that a readable style contributes to the readers’perseverance, also called depth or persistence, the tendency to keep reading thetext.
Charles E. Swanson (1948) showed that better readability increases reading
perseverance as much as 80 percent. He developed an easy version of a story
with 131 syllables per 100 words and a hard version with 173 syllables anddistributed each to 125 families. A survey of readers taken 30 hours after
distribution showed a gain in the easier version over the hard version of 93% of
total paragraphs read, 83% in mean number of paragraphs read, and 82% in thenumber of correspondents reading every paragraph.
Bernard Feld (1948) grouped 101 stories from the Birmingham News into thosewith high Flesch scores, requiring 9th-grade education or more and those withlow scores, requiring less than 9th-grade education. He found readershipdifferences of 20 to 75 percent favoring the low-score versions. Feld’s findings
indicated that even a small actual percentage gain for a large-circulation paper
greatly increased the number of readers.
Reading efficiency Klare, Shuford, and Nichols (1957) followed up thesestudies with a study of the reading efficiency and retention of 120 male aviators
in a mechanics course at Chanute Air Force Base in Illinois. They used two
versions of technical training materials, hard (13th-15th grade) and easy (7th-8th
grade).
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The Principles of Readability

They measured reading efficiency with an eye-movement camera with which
they could determine the number of words read per second and the number ofwords read per fixation. A strong “set-to-learn” was stimulated by allowing the
subjects to re-read the text and giving them a pre-test before the experimental
test.
The study showed that the easy text significantly improved both reading
efficiency and retention. The results also indicated that a strong “set to learn”
improved scores.
Hardyck and Petrinovich (1970) showed the connection between readability andboth comprehension and muscle activity in the oral area (subvocalization).
Rothkopf (1977) showed the connection between readability and how many
words a typist continues to type after the copy page is covered (functional
chaining).
Readability and course completion Publishers of correspondence courses areunderstandably concerned when large numbers of students do not complete thecourses. They often suspect the materials are too difficult for the students.
Working with Kim Smart of the U. S. Armed Forces Institute, Klare (1973)
applied the Flesch Reading Ease formula to thirty sets of printed correspondence
courses used by the military.
They found that two of the high school courses and five of the college courses
were too difficult for readers of average or below average reading skill.
They then compared their reading analysis to the completion records of the 17
courses that had been in use over two years. They found a Spearman rank-ordercorrelation of .87 between the readability score and the probability of students
completing the course. There was a Pearson product-moment correlation of .76.
These results showed the importance of readability for unassisted reading wherepressure to complete a course of study is low and competition from distractions
is high.

The Measurement of Content
For hundreds of years, writers and teachers have used and taught the cognitiveand structural factors in text such as organization and coherence. Researchers inreadability also addressed the effects of these factors on comprehension:•

Image words, abstraction, predication, direct and indirect discourse,
types of narration, and types of sentences, phrases, and clauses (Grayand Leary 1935).

•

Difficult concepts (Morriss and Holverson 1938, Chall 1958).

•

Idea density (Dolch 1939).

•

Human interest (Flesch 1949, Gunning 1952)•

Organization (Gunning 1952, Klare and Buck 1954, Chall 1958).

•

Nominalization (Coleman and Blumenfeld 1963; Coleman, 1964)•

Active and passive voice (Gough 1965, Coleman 1966, Clark andHaviland 1977, Hornby 1974).

•

Embeddedness (Coleman 1966).

The cognitive theorists and linguists, beginning in the 1970s, promoted the ideathat reading was largely an act of thinking. Among the ideas they promoted
were:1.
Copyright © 2004 William H. DuBay

Meaning is not in the words on the page. The reader constructs
meaning by making inferences and interpretations.

Page 31The Principles of Readability

2.

Information is stored in long-term memory in organized "knowledge
structures." The essence of learning is linking new information to priorknowledge about the topic, the text structure or genre, and strategies
for learning.

3.

A reader constructs meaning using metacognition, the ability to think
about and control the learning process (i.e., to plan, monitor
comprehension, and revise the use of strategies and comprehension);and attribution, beliefs about the relationship among performance,
effort, and responsibility (Knuth and Jones 1991).

The cognitive theorists, aware of the limitations of the readability formulas, set
about to supplement them with ways to measure the content, organization, andcoherence of the text. Their studies reinforced the importance of these variables
for comprehension. They did not, however, come up with any practical method
for measuring or adjusting them for different levels of readers.
The following sections summarize a few of these efforts.
Walter Kintsch and coherence Beginning in 1977, Walter Kintsch and his
associates studied the cognitive and structural issues of readability. Kintsch
proposed to measure readability by measuring the number of propositions in a
text. A proposition consists of a predicate and one or more arguments. An
argument can be a concept or another argument. A concept is the abstract idea
conveyed by a word or phrase.
In the early part of his work, Kintsch (Kintsch and Vipond 1979) was quite
critical of the readability formulas. He said they are not based on modern
linguistic theory and they overlook the interaction between the reader and thetext.
Over four years, however, he and his associates revised this position. He
eventually admitted that “these formulas are correlated with the conceptual
properties of text” and that vocabulary and sentence length are the strongest
predictors of difficulty (Kintsch and Miller 1981, p. 222).
While Kintsch and his colleagues did not come up with any easily used formula,they did contributed to our understanding of readability, including the central
role of coherence in a text. Kintsch found out that lack of coherence affects
lower-grade readers much more than upper-grade ones. The upper-grade readers,in fact, feel challenged to reorganize the text themselves. They may require more
opportunities for solving problems, while lower-grade readers require more
carefully organized texts.
The Air Force transformational formula. Perhaps the most ambitious attempt
to quantify the variables of the cognitive theorists and put them in a formula was
the project of Williams, Siegel, Burkett, and Groff (1977). Working for the Air
Force Human Resources Laboratory, they examined new variables, produced a
new formula, and presented supporting data. The variables they included were:•

Four psycholinguistic variables such as Yngve word depths,transformational complexity, center embedding, and right branching.

•

Four Structure of Intellect variables including cognition of semantic
units, memory for semantic units, evaluation of symbolic implications,and divergent production of semantic units.

For a criterion, they used cloze scores on 14 passages of about 600 words eachtaken from the Air Force career-development course. They deleted each tenthword in the cloze test and used only one version out of a possible ten on 51 AirForce subjects. The computerized formula produced a correlation of 0.601 withtext difficulty.
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The Principles of Readability

Susan Kemper and the reader’s mental load Following Kintsch, Susan
Kemper (1983) sought to explain comprehension in terms of underling cognitive
processes. She developed a formula designed to measure the “inference load”
based on three kinds of causal links:•

Physical states•

Stated mental states•

Inferred mental states

The Kemper formula measures the density of the propositions and embedded
clauses. It takes considerable time and effort in comparison to the readability
formulas. It has a correlation of .63 with the McCall-Crabbs tests (the original
Dale-Chall formula has a correlation of .64).
Kemper (p. 399) commented: “..sentence length and word familiarity docontribute to the comprehension of these passages…. These two different
approaches to measuring the grade level difficulty of texts are equivalent inpredictive power.”
Kemper admitted that her formula, like all readability formulas, is better at
predicting problems than fixing them. For writing, both formulas are best used asa general guide.
Bonnie Meyer and organization Bonnie Meyer and others worked on using
the organization of larger units of texts as a possible measurement of readability.
She claimed that a text that follows a topical plan is more efficient (saves effort)
and more effective (gets more results). She wrote:
That is, people remember more and read faster information which is
logically organized with a topical plan than they do when the same
information is presented in a disorganized, random fashion…. Thus
the plan of discourse can be considered apart from content, anddeserves separate consideration from researchers, as from those whoare planning a composition (Meyer 1982, p. 38).
Among Meyer’s observations are the following:•

A visible plan for presenting content plays a key role in assessing thedifficulty of a text.

•

A plan incorporates a hierarchy showing the dependencies of the facts
to one another:• The antecedent/consequent plan shows causal relationships in“if/then” logic.
• The comparison plan presents two opposing views that give weight toboth sides.
• The adversative plan clearly favors one side over the other (politicalspeeches).
• The description plan describes the component parts of an item
(newspaper articles). This plan is the least effective for rememberingand recall.
• The response plan gives answers to remarks, questions, and problems(science articles).
• The time-order plan relates events chronologically (history texts).
Better readers tend to share the same plan as authors of the material they arereading. Readers who use a different plan other than the authors may be at a
disadvantage.
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The Principles of Readability

There are two types of highlighting for showing the relationships between items:•

Subordination, used to connect the main idea with supporting text as
in a hierarchical structure.

•

Signaling, explicit markers to clarify relationships such as:“On the one hand…On the other hand…”“Three things have to be stressed here.”“Thus,” “consequently,” and “therefore”“Nevertheless,” “all the same,” “although,” “but,” and “however”

Signaling can also clarify how larger blocks of content are related, for example:“For example,” “For further details,” “summary,” “abstract,” “conclusion,” and“preview.” For more on signaling, see the studies by Jan Spyridakis (1989,
1989a).
Besides reducing the difficulty of the text, Meyer wrote that strategy training canalso help older adults deal with the difficulties they encounter in reading.
Bonnie Armbruster and textual coherence Also concerned with larger unitsof text, Bonnie Armbruster (1984) found that the most important feature forlearning and comprehension is textual coherence, which comes in two types:•

Global coherence, which integrates high-level ideas across an entire
section, chapter, or book.

•

Local coherence, which uses connectives to link ideas within andbetween sentences.

Armbruster found that recalling stories from memory is superior when the
structure of the story is clear. She also noted the close relationship between
global content and organization. Content is an aspect of structure, andorganization is the supreme source of comprehension difficulty.
For local coherence, Armbruster stressed the highlighting that carries meanings
from one phrase, clause, or sentence to another:••••

Pronoun references to previous nouns
Substitutions or replacements for a previously used phrase or clause(sometimes called “resumptive modifiers”), for example: “These results[previously listed] suggest that…”
Conjunctions
Connectives

Finally, Armbruster supported Kintsch’s finding that coherence and structure are
more important for younger readers than older ones, simply because they haveless language and experience.
Calfee, Curley, and the familiar outline R.C. Calfee and R. Curley (1984)
built on the work of Bonnie Meyer. They stressed making the structure of thetext clear to upper-grade readers. The content can be simple, but an unfamiliar
underlying structure can make the text unnecessarily difficult.
They proposed that the teacher, researcher, and student all need to reach amutual understanding of the type of outline being used for the text underdiscussion.
Most students are familiar with the narrative structure, but not with other forms.
Calfee and Curley present a graduated curriculum that enables students toprogress from simpler structures to ones that are more difficult:1.
Copyright © 2004 William H. DuBay

Narrative—fictional and factual

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The Principles of Readability

2.

Concrete process—descriptive and prescriptive

3.

Description—fictional, factual particular, and factual general

4.

Concrete topical exposition

5.

Line of reasoning—rational, narrative, physical and relational causeand-effect6.

Argument—dialogue, theories and support, reflective essay

7.

Abstract exposition

The lessons of content, organization, and coherence Organization andcoherence highlight the relationships between words, sentences, paragraphs, andlarger sections of text. They enable readers to fit new items of information into
their own cognitive systems of organization.
The cognitive studies of readability also showed other problems that texts canreveal or create, such as:•Unfamiliar life experiences and background•The need for time to digest illustrations and new material•

The need for multiple treatments of difficult material•

The need for learning aids to overcome textual difficulty•

The need for learning aids to help readers of different levels of skill.

Generally, however, the cognitive researchers failed to translate their theories
into practical and objective methods for adjusting the difficulty of texts fordifferent levels of reading skill.
Critics of the formulas (e.g., Manzo 1970, Bruce et al. 1981, Selzer 1981,
Redish and Selzer 1985, Schriver 2000) rightly claim that the formulas use only
“surface features” of text and ignore other features like content and organization.
The research shows, however, that these surface features—the readability
variables—with all their limitations have remained the best predictors of text
difficulty as measured by comprehension tests (Hunt 1965, Bormuth 1966,
Maxwell 1978, Coupland 1978, Kintsch and Miller 1981, Chall 1984, Klare
1984, Davison 1984 and 1986, Carver 1990, Chall and Conard 1991, Chall andDale 1995).

Text LevelingAn important byproduct of the cognitive and linguistic emphasis was therenewed interest in text leveling. This involves a subjective analysis of readinglevel that examines vocabulary, format, content, length, illustrations, repetitionof words, and curriculum. Text leveling is perhaps the oldest method of gradinga text. The McGuffey readers were graded by leveling, and their success is anindication of its validity.
Leveling recently became popular largely due to the work of the New ZealandDepartment of Education. In the U.S., Marie Clay’s (1991) Reading Recovery
system uses leveling in tutoring of children with reading problems. In thissystem, teachers use leveling to find books with closely spaced difficulty levels,
particularly at the first-and second-grade levels. Most traditional readability
formulas are not particularly sensitive at those levels (Fountas and Pinnell,1999).
For that same reason, readability experts have long encouraged the use of
subjective leveling along with the readability formulas. Leveling can spot theitems that the formulas do not measure (Klare 1963, pp. 137-144; Chall et al.
1996; Fry 2002).
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The Principles of Readability

R. P. Carver (1975-1976) introduced a method of using qualified raters toassess the difficulty of texts. Raters become qualified when accurately judging
the difficulty of five passages using his “Rauding Scale,” consisting of six
passages representing grades 2, 5, 8, 11, 14, and 17. Carver claimed his method
was slightly more accurate than the Dale and Chall and Flesch Reading Ease
formulas and provides grade-level scores through grade 18.
H. Singer (1975) created a method called SEER, “Singer Eyeball Estimate of
Readability.” It involves the use of one or two accurate SEER judges matching asample of text against one of two scales, each consisting of eight rated passages.
Singer claims his method is as accurate as the Fry graph.
The problem, of course, is that it takes some effort to learn how to do levelingaccurately. Advanced readers often fail to recognize how difficult texts can befor others. Leveling also becomes more effective and accurate as the number ofexperienced judges increases (Klare, 1984).
Jeanne Chall and her associates (1996) published Qualitative Assessment of Text
Difficulty, A Practical Guide for Teachers and Writers. It uses graded passages,
called “scales,” from published works along with layouts and illustrations forleveling of texts. You can assess the readability of your own documents bycomparing them to these passages and using the worksheet in the book. The 52
passages are arranged by grade level and by the following types of text:•

Literature•

Popular fiction•

Life sciences•

Physical sciences•

Narrative social studies•

Expository social studies

The scale passages were selected on the basis of the following grade-related
requirements for the reader:1.

Knowledge of vocabulary

2.

Familiarity with sentence structure

3.

Subject-related and cultural knowledge

4.

Technical knowledge

5.

Density of ideas

6.

Level of reasoning

The selections were then tested by:1.

Evaluation by several groups of teachers and administrators

2.

Evaluation by students of corresponding grades

3.

Cloze testing of students of corresponding grades

4.

Readability formulas (Dale-Chall and Spache)

The book also describes at length the various characteristics of each type of textthat can contribute to difficulty. An added section features samples of the design
and illustrations of books appropriate for the first four grades.
The following are three samples of the scales taken from the book.

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The Principles of Readability

Reading Level 3
The stars, like the sun, are always in the sky, and they are always
shining. In the daytime the sky is so bright that the stars do not show.
But when the sky darkens, there they are.
What are the stars, you wonder, and how do they twinkle?
Stars are huge balls of hot, hot gas. They are like the sun but they looksmall because they are much, much farther away. They are trillions andtrillions of miles away, shining in black space, high above the air.
Space is empty and does not move. Stars do not twinkle there, but
twinkling begins when starlight hits the air. The air moves and tosses
the light around.
—From The Starry Sky: An Outdoor Science Book (Wyler 1989, pp. 1516)
Reading Level 5-6Black holes are probably the weirdest objects in space. They are
created during a supernova explosion. If the collapsing core of the
exploding star is large enough—more than four times the mass of oursun—it does not stop compressing when it gets as small as a neutronstar. The matter crushes itself out of existence. All that remains is thegravity field—a black hole. The object is gone. Anything that comesclose to it is swallowed up. Even a beam of light cannot escape.
Like vacuum cleaners in space, black holes suck up everything aroundthem. But their reach is short. A black hole would have to be closer
than one light-year to have even a small effect on the orbits of theplanets in our solar system. A catastrophe such as the swallowing of
the Earth or the sun is strictly science fiction.
—From Exploring the Sky (Dickinson 1987, p. 42)
Reading Level 7-8
As we have seen, a neutron star would be small and dense. It should
also be rotating rapidly. All stars rotate, but most of them do soleisurely. For example, our Sun takes nearly one month to rotate
around its axis. A collapsing star speeds up as its size shrinks, just as
an ice-skater during a pirouette speeds up when she pulls in her arms.
This phenomenon is a direct consequence of a law of physics known as
the conservation of angular momentum, which holds that the total
amount of angular momentum in a system holds constant. An ordinary
star rotating once a month would be spinning faster than once a second
if compressed to the size of a neutron star.
In addition to having rapid rotation, we expect a neutron star to have an
intense magnetic field. It is probably safe to say that every star has amagnetic field of some strength.
—From Discovering the Universe (Faufmann 1990, p. 290)

Producing and Transforming Text
While the formulas were originally created to help educators select texts fordifferent audiences, writers also use the formula variables to produce texts andtransform (re-write) them into simpler versions. The evidence on how effective
this is has been mixed. As both the supporters of the formulas and their critics
have warned, if you just chop up sentences and use shorter words, the results are
not likely to improve comprehension. You have to look at the many other factorsthat affect reading at the level for which you are writing.
Early evidence on the effects of using the formula variables to transform text was
negative. Klare (1963) reported that, of the six readability studies involving thecontrolled manipulation of words or sentences, only one had a positive effect,
and this involved simplifying vocabulary.
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The Principles of Readability

In a later study, Klare (1976) took a careful look at 36 studies that examined theeffects on comprehension of using the readability formula variables in re-writing
texts. He grouped them by their results:•

19 studies had positive results (readability variables had a significant
effect on comprehension and/or retention)•

6 studies had mixed results•

11 studies had negative results (no measurable effect).

In seeking the reasons for the differences, Klare looked carefully at 28
situational factors in which each experiment was conducted. The situational
factors fell into these groups:•

The readability and content of the material.

•

The competence and motivation of the subjects.

•

The instructions given the subjects during the experiment.

•

The details of the test situation.

Klare found that differences in readability were often overridden by other factors
in the test situation such as:•

The instruction given to the subjects of the test.

•

The presence of threats or rewards.

•

The time allowed for reading and testing.

•

The presence or absence of the text during the test.

Klare wrote that the performance of the subject in such tests is a function not
only of the difficulty of the material, but also in critical degrees, a function of thetest situation (time, place, etc.), the content of the material and the competence
and motivation of the reader. Scores will be better, for instance, when the readers
love the subject matter or if they are highly motivated (e.g., paid).
Klare concluded that in the studies that showed increased comprehension,transforming text requires attending to other problems besides word andsentence length. “The best assumption, it seems to me,” he wrote, “is that the
research workers, probably with considerable effort, managed to change basic
underlying causes of difficulty in producing readable versions” (p. 148). Klare
then listed the following word-and-sentence variables that affectedcomprehension:
Word characteristics:
1.

Proportion of content (functional) words.

2.

Frequency, familiarity, and length of content words.

3.

Concreteness or abstractness.

4.

Association value.

5.

Active vs. nominalized verb constructions.

Sentence characteristics:

Copyright © 2004 William H. DuBay

1.

Length (esp. clause length).

2.

Active vs. passive.

3.

Affirmative vs. negative.

4.

Embedded vs. non-embedded.

5.

Low depth vs. high depth (branches).

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The Principles of Readability

Since Klare’s 1976 study, there have been other studies showing the positiveeffects of using formula variables to improve comprehension (Ewing 1976,
Green 1979, C. C. Swanson 1979).
In the many studies of before-and-after revision of the text, a negative result does
not prove that there is no improvement in comprehension. They show insteadthat improvement has not been detected. There is a saying in statistics that you
cannot prove a negative.
Studies reporting a negative result may result from failing to control the reading
ability, prior knowledge, interest, and motivation of the subjects. They can also
result from failing to control elements of the text such as organization,
coherence, and design. The great difficulty of properly conducting such an
experiment is seen in following two studies.
The Duffy and Kabance study Critics worry that technical communicators can
too easily misuse the formulas, making documents more difficult, not less(Charrow 1977, Kern, 1979, Selzer 1981, Lange 1982, Duffy 1985, Redish andSelzer 1985, Connaster 1999, Redish 2000, Schriver 2000). These writers offer
little or no evidence of such misuse, however, widespread or otherwise. If
unscrupulous or careless writers choose to cheat by “writing to the formula” andnot attending to other textual issues, careful editors and reviewers easily spot themisuse. The study by Thomas Duffy and Paula Kabance (1981) is a case inpoint. Because formula critics (e.g., Redish and Selzer 1985; Redish 2000) often
refer to this study, it deserves some attention.
The Duffy and Kabance study consisted of four experiments that examined theeffects of changing only word and sentence length on comprehension. It used a“reading to do” task and a “reading to learn” task. The study used four versionsof the text:
1.

The original version (a narrative or expository passage from the 1973
Nelson-Denny reading tests).

2.

One with vocabulary that they simplified using The Living Word
Vocabulary3.

One with only simplified and shortened sentences.

4.

One with both vocabulary and sentences simplified.

The effect was a 6-grade drop in reading level of the changed passages from the11th to the 5th grade.
Following Klare’s research protocols (1976), they attempted to maximize thereadability effects by using readers who were low motivated, unfamiliar with the
topic, and widely varying in reading skills.
Using the Nelson-Denny reading tests, they tested the reading ability of the
1,169 subjects, male Navy trainees between 17 and 20 years old, of which 80%
were high-school graduates. They divided them into two groups, one with a
median reading grade of 8.7 and the other 10.3. The experiments took place ingroups of 40 to 70.
In the first two experiments, they simulated a “reading-to-do” situation. In thefirst experiment, they first showed the questions, then had the subjects read thetext. After that, they were shown the questions again, which they answered. In
the second experiment, they limited the reading time but let the subjects haveaccess to the text while answering the questions. The third experiment was astandard cloze test. The fourth experiment was a standard multiple-choice testwith the subjects first reading the text and then answering the questions without
the text.
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The Principles of Readability

The first three experiments showed no significant improvements. The fourthexperiment resulted in significant improvement but only with the low-ability
group using the changed-vocabulary text, an improvement of 13 percent. The
authors concluded that simplifying the text made no difference to the advanced
readers. This is not a surprising result, when we consider the reading ability of
the advance group was at grade 10.3 while the difficult text was at 11th grade.
The vocabulary variable is significant for the low-ability group, they stated, but
only in reading-to-learn tasks but not reading-to-do tasks, where memory is lessimportant. This correlation was also suggested by Fass and Schumacher (1978).
Duffy and Kabance concluded that the increased readability is not required fortechnical documents, in which the emphasis is on “reading to do” and memory is
not required.
This is sometimes true. At other times, serious errors have taken place becauseof memory failure. Many, if not most, technical tasks involve learning a skill thatcan be repeated, as Redish (1988) emphasizes. Besides reading-to-learn andreading-to-do tasks, she writes, many technical tasks require “reading to learn todo.” Technical texts may require more memory than do most other kinds ofliterature such as magazines, newspapers, or fiction.
When we look at the methods of these experiments, difficulties appear that
explain their inconsistent results. In their report, Duffy and Kabance provide four
sample passages used in the study. The re-written passages appear disjointed andstilted, not what one would expect of a 5th-grade text (See Fig. 11). If thesestudies are representative of the other passages, we must assume that judges were
not used to control for the coherence and content of the text.
This was the also the conclusion of Leslie Olsen and Rod Johnson (1989), whowrote: “In their study, Duffy and Kabance were trying to directly manipulate theunderstanding of the words and the syntax of the sentences. However, it seemed
to us that they were also unintentionally altering other aspects of the text—inparticular, the cohesive structures of the text.”
In their paper, Olsen and Johnson defined “sensed cohesion” as the strength of
the textual topicality and the sense of givenness. The strength of textual
topicality is related to the persistence of what the text is about. The sense of
givenness is the recognition that the reader has seen a particular noun phrase
before.
In analyzing the passages of the Duffy and Kabance study, Olsen and Johnson
found that long sentences were broken up into short sentences. In the process,they introduced new subjects. The original focus on the Spaniards was lost,
making it difficult to know what the text is about. They analyzed thecohesiveness of the text and concluded, “the intended and the unintended effectsof the revisions cancelled one another out,” bringing the results of the study into
question.

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The Principles of Readability

Original (11th Grade)

Sentences and Vocabulary Revised (5th Grade)

The night was cloudy, and a drizzling rain, which fell
without intermission, added to the obscurity. Steadily,
and as noiselessly as possible, the Spaniards made their
way along the main street, which had so lately
resounded to the tumult of battle. All was now hushed
in silence; they were only reminded of the past by theoccasional presence of some solitary corpse, or a darkheap of the slain, which too plainly told where the strife
had been the hottest. As they passed along the lanes andalleys which opened into the great street, they easily
fancied they discerned the shadowy forms of their foe
lurking in ambush, ready to spring upon them. But it
was only fancy; they city slept undisturbed even by the
prolonged echoes of the tramp of the horses, and thehoarse rumbling of the artillery and baggage trains. At
length, a lighter space beyond the dusky line ofbuildings showed the van of the army that it was
emerging on an open causeway. They might well havecongratulated themselves on having thus escaped thedangers of an assault in the city itself, and that a brieftime would place them in comparative safety on the
opposite shore.

The night was cloudy. A sprinkling rain added to thedarkness. It fell without a break. The Spaniards madetheir way along the main street. They moved without
stopping and with as little noise as possible. The street
had so recently roared to the noise of battle. All was now
hushed in silence. The presence of a single dead bodyreminded them of the past. A dark heap of the slain also
reminded them. Clearly, the battle had been worse there.
They passed along the lanes and alleys opening into thegreat street. They easily fancied the shadows of theirenemy lying in wait. The enemy looked ready to spring
upon them. But it was only fancy. The city slept without
being bothered by the rough rumbling of the cannons
and baggage trains. Even the constant sound of the trampof horses did not bother the city. At length, there was abright space beyond the dark line of the buildings. This
informed the army look-out of their coming out onto theopen highway. They might well have rejoiced. They had
thus escaped the dangers of an attack in the city itself. A
brief time would place them in greater safety on the
opposite shore.

Fig. 10. Original and revised samples of the passages used in the Duffy and Kabance study of 1981. Lack ofattention to coherence and other important variables can cancel out the effects of rewriting the text using thereadability-formula variables.
The Charrow and Charrow study Critics of the formulas (e.g., Bruce et al.
1981, Redish and Selzer; Redish 2000) also refer to the elaborate study of oral
jury instructions by attorney Robert Charrow and linguist Veda Charrow (1979).
They claimed that simplifying text did not make verbal instructions morecomprehensible.
The authors did not use the readability variables in re-writing jury instructions
but simplified the instructions using a list of common legal “linguistic
constructions.” These were: nominalizations, unusual prepositional phrases,
misplaced phrases, whiz deletions (use of participles instead of verbs), deletionsof “that” or “which” beginning dependent clauses, technical legal terms,
imperative terms, negatives, passive voice, word lists, organization, anddependent clauses.
The first experiment used 35 persons called for jury duty in Maryland using 14
jury instructions taken from California’s standard civil jury instructions. The
purpose of the study was mainly to see if it was the complexity of the legal issues
that made the instructions difficult or the difficulty of the language used. A groupof attorneys were asked to rate the instructions according to their perceived
complexity.
The experimenters tested each person individually by playing each instruction
twice on a tape recorder. After hearing each instruction, the subject then verbally
paraphrased the instruction, which was also recorded. The results showed,
contrary to the attorneys’ expectations, it was not the complexity of the ideas thatcaused problems in comprehension, but the difficulty of the language.

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The Principles of Readability

The second experiment used 48 persons chosen for jury duty in Maryland. For
this experiment, they re-wrote the instructions, paying close attention to the legal
constructions noted above. They divided the group into two. Using 28 original
and modified instructions, they gave seven original instructions and sevenmodified instructions to each group. They used the same protocols in playing the
instructions twice and asking the subjects to paraphrase them.
There was a significant improvement of the mean scores in comprehension innine of the fourteen instructions. They concluded that the subjects understood
the gist of the original only 45% of the time and the simpler ones 59% of thetime.
This is not good enough, according to Professor Robert Benson (1984-85) of
Loyola Law School in Los Angeles. He wrote, “…none of us would care to betried by jurors who understood only 59% of their instructions.”
Benson went on to say that the Charrows own data was leading them to aconclusion that they were unable to draw: that juries are never likely tounderstand oral instructions adequately. Elwork, Sales, and Alfini (1982) reachthe same conclusion and recommend giving all jurors written as well as oral
instructions.
To prove his point, Benson included three of the Charrows’ re-written
instructions in his own study on legal language using 90 law students and 100
non-lawyers. Using cloze tests, he found that, while the Charrows had reported
59% comprehension, the readers understood the written instructions almost fully(p. 546).
As to the claim that paraphrasing is better than other testing techniques, Benson
claims that it has its own limitations, depending as it does on the subjects’ ability
to orally articulate what they understand. The Charrows had avoided asking the
subjects to paraphrase in writing because “subject’s writing skills would
confound the results.” Unfortunately, they ignored similar possible differences intheir listening and their oral skills (Benson, p. 537).
The Charrows state that sentence length does not cause reading difficulty.
“Although readability formulas are easy to use,” they write, “and certainly do
indicate the presence of lengthy sentences, they cannot be considered measuresof comprehensibility. Linguistic research has shown that sentences of the samelength may vary greatly in actual comprehensibility” (p. 1319).
Benson answered by writing that extremely long sentences such as those foundin legal language are known to cause difficulty, probably because of memory
constraints. He also found that the Charrows’ revised instructions had actuallyshortened sentences by 35 percent. The shorter sentences “may well have played
a role in improved comprehension” (pp. 552-553).
A number of studies show that, in the average, as a sentence increases in length itincreases in difficulty (e.g., Coleman, 1962, Bormuth 1966). Average sentence
length has long been one of the clearest predictors of text difficulty.

Copyright © 2004 William H. DuBay

Page 42The Principles of Readability

The New Readability Formulas
Critics of the formulas and formula developers questioned the reliability of thecriterion passages, criterion scores, and the reading tests on which the formulas
had been developed and validated. The arrival of cloze testing stimulated thedevelopment of new criterion passages, new formulas, manual aids,
computerized versions, and the continued testing of text variables.
The Coleman formulas Edmund B. Coleman (1965), in a research projectsponsored by the National Science Foundation, published four readability
formulas for general use. The formulas are notable for their predicting mean
close scores (percentage of correct cloze completions).
Coleman was also the first to use cloze procedures as a criterion rather than theconventional multiple-choice reading tests or rankings by judges.
The four formulas use different variables as shown here:
C% = 1.29w – 38.45
C% = 1.16w + 1.48s – 37.95
C% = 1.07s + 1.18s + .76p – 34.02
C% = 1.04w + 1.06s + .56p – .36prep – 26.01
Where: C% = percentage of correct cloze completions;w = number of one-syllable words per 100 wordss = number of sentences per 100 words
p = number of pronouns per 100 words
prep = number of prepositions per 100 words
Coleman found multiple correlations of .86, .89, .90, and .91, respectively, for
his formulas with cloze criterion scores. The use of cloze scores as criterion
consistently provides higher validation coefficients than does use of the multiplechoice scores. This may be a partial reason for the high correlations shown here.
The Bormuth studies Recognizing the problems of having more reliable
criterion passages, John Bormuth conducted several extensive studies, whichgave a new empirical foundation for the formulas. His first study (1966)
provided evidence of just how much changes in a number of readability variables
beside just vocabulary and sentence length can affect comprehension. Cloze
testing made it possible to measure the effects of those variables not just on the
difficulty of whole passages but also on individual words, phrases, and clauses.
His subjects included the entire enrollment of students (675) in grades 4 through8 of Wasco Union Elementary School district in California. Their reading levels
went from the 2nd through the 12th grade. He used 20 passages of 275 to 300
words each, rated on the Dale-Chall formula from the 4th to the 8th-grade levelsof difficulty. He used five cloze tests for each passage, with the fifth-worddeletions starting at different words.
Reading researchers recognized that beginning readers relate differently to word
variables than do better readers. For this reason, special formulas have beendeveloped for the earliest primary grades such as the Spache formula (1953) andthe Harris-Jacobson primary readability formula (1973).
Bormuth’s study confirmed the curvilinearity of the formula variables. Thatmeans their correlation with text difficulty changes in the upper grades,
producing a curve when plotted on a chart. Dale and Chall (1948) included anadjustment for this feature in their formula-correction chart. This adjustment wasalso included in the SMOG formula (McLaughlin 1968), the Fry Graph (FryCopyright © 2004 William H. DuBay

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The Principles of Readability1969), the FORCAST formula (Caylor et al. 1973), Degrees of Reading Progress(Koslin et al. 1987), and the ATOS formula (Paul 2003).
Some critics of the formulas (Rothkopf 1972, Thorndike 1973-74, Selzer 1981,
Redish and Selzer 1985) claim that decoding words and sentences is not a
problem for adults. Bormuth’s study, however, shows that the correlation
between the formula variables and comprehension do not change as a function ofreading ability (p. 105). Empirical studies have confirmed that, in adult readers,difficulty in reading is linked to word recognition (Stanovich 1984) anddecoding of sentences (Massad 1977). We cannot assume that adults are better
learners than children of the same reading level. In fact, they are often worse(Russell 1973, Sticht 1982).
Bormuth’s next project (1969) was a study of the readability variables and theirrelationship to comprehension. His subjects included 2,600 fourth-to-twelfthgrade pupils in a Minneapolis school district.
The method consisted first in rating the reading ability of all the students with
the California 1963 Reading Achievement test. It used 330 different passages ofabout 100 words each to confirm the reliability of 164 different variables, manyof them never examined before such as the parts of speech, active and passive
voice, verb complements, and compound nouns.
The five cloze tests used for each passage (resulting in 1,650 tests) gave him
about 276 responses for each deleted word, resulting in over 2 million responses
to analyze.
With this data, Bormuth was able to develop 24 new readability formulas, someof which used 14 to 20 variables. These new variables, he found, added little tothe validity of the two classic-formula variables and were eventually dropped.
The study divided the students of each reading level into two groups, one thatwas given a multiple-choice test and the other a cloze test of the same material.
Since Thorndike’s (1916) recommendation, educators and textbook publishers
had used 75% correct scores on a multiple-choice test as the criterion for
optimum difficulty for assisted classroom learning, and 90% for independent
reading. These criterion scores, however, were based on convention and use, noton scientific study.
This Bormuth study validated the equivalencies of 35%, 45%, and 55% correctcloze criterion scores with 50%, 75%, and 90% correct multiple-choice scores. It
also showed that the cloze score of 35% correct answers indicates the level ofdifficulty required for maximum information gain.
Finally, this study produced three different formulas, one is for basic use, one formachine use, and one for manual use. All three formulas predict the difficulty oftexts for all grade levels using a 35%, 45%, 55%, or a mean-cloze criterion.
The Bormuth Mean Cloze formula This formula uses three variables: numberof words on the original Dale-Chall list of 3,000, average sentence length inwords, and average word length in letters. This formula was later adapted andused in the Degrees of Reading Power used by the College Entrance
Examination Board in 1981 (see below).

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Page 44The Principles of Readability

The original Bormuth Mean Cloze formula is:
R = .886593 – .083640 (LET/W) + .161911 (DLL/W)3– 0.021401 (W/SEN) + .000577 (W/SEN)2– .000005 (W/SEN)3
DRP = (1 – R ) x 100
Where: R = mean cloze score
LET = letters in passage X
W = words in passage X
DLL = Number of words in the original Dale-Chall list in
passage X
SEN = Sentences in passage X
DRP = Degrees of Reading Power, on a 0-100 scalewith 30 (very easy) to 100 (very hard)
The findings of Bormuth about the reliability of the classic variables wereconfirmed by MacGinitie and Tretiak (1971) who said that the newer syntactic
variables proposed by the cognitive theorists correlated so highly with sentence
length that they added little accuracy to the measurement. They concluded that
average sentence length is the best predictor of syntactic difficulty.
The Bormuth studies provided formula developers with a host of new criterion
passages. Critics of the formulas claimed that the criterion passages used byformula developers were arbitrary or out-of-date (Bruce et al. 1981, Duffy,1985). As new criterion passages became available, developers used them tocreate new formulas and to correct and reformulate the older ones (Bormuth
1966, 1969, Klare 1985). The new Dale-Chall formula (1995) was validated
against a variety of criterion passages, including 32 developed by Bormuth(1971), 36 by Miller and Coleman (1967), 12 by Caylor et al. (1973) and 80 by
MacGinitie and Tretiak (1971). Other formulas were validated against normed
passages from military technical manuals (Caylor et al. 1973, Kincaid et al.
1975).
The Fry Readability Graph While Edward Fry (1963, 1968) was working as a
Fullbright scholar in Uganda trying to help teachers teach English as a second
language, he created one of the most popular readability tests that use a graph.
Fry would go on to become the director of theReading Center of Rutgers University and an
authority on how people learn to read.
Fry’s original graph determines readability
through high school. It was validated withcomprehension scores of primary and secondary
school materials and by correlations with otherformulas.

Fig. 11. Edward Fry. His
Readability Graph may be themost popular readability aid.

Copyright © 2004 William H. DuBay

Fry (1969) later extended the graph to primarylevels. In 1977, he extended it through the college
years (Fig. 11). Although vocabulary continues toincrease during college years, reading ability
varies much, depending on both individuals andthe subjects taught. That means that a text with ascore of 16 will be more difficult than one with ascore of 14. It does not mean, however, that one
is appropriate for all seniors and the other for all
sophomores.

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The Principles of Readability

Directions:1.

Select samples of 100 words.

2.

Find y (vertical), the average number of sentences per 100-wordpassage (calculating to the nearest tenth).

3.

Find x (horizontal), the average number of syllables per 100-wordsample.

4.

The zone where the two coordinates meet shows the grade score.

Fig. 12. The Fry Readability Graph as amended in 1977 with the extension
into the primary and college grades. Scores that appear in the dark areas
are invalid.

The Listening Formulas People have been concerned about the clarity of
spoken language perhaps for a longer period than written language. Speech is
generally much simpler than text. Because a listener cannot re-read a spoken
sentence, it puts a greater demand on memory. For this reason, “writing like youtalk” and reading text aloud have long been methods for improving readability.
Studies of the correlations of listenability and readability have had mixed results(Klare 1963).
Some formulas have been developed just for spoken text. Rogers (1962)
published a formula for predicting the difficult of spoken text. He used 480
samples of speech taken from the unrehearsed and typical conversations ofstudents in elementary, middle, and high school as his data for developing his
formula. The resulting formula is:
G = .669 I + .4981 LD – 2.0625
Where:
G = reading grade level
I = average idea unit length
LD = the average number of words in a hundred-word sampling that do notappear on Dale’s long list (3,000 words).
Roger’s formula has a multiple correlation of .727 with the grade level of his
samples.

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The Principles of Readability

Irving Fang (1966-1967) used newscasts to develop his Easy Listening Formula(ELF), shown here:
ELF = number of syllables above one per word in a sentence.
An average sentence should have an ELF score below 12 for easy listenability.
Fang found a correlation of .96 between his formula and Flesch’s Reading Ease
formula on 36 television scripts and 36 newspaper samples.
Subsequent research into listenability indicates that after the 8th grade, listening
skills do not keep up with the improvement in reading skills. After the 12th-gradelevel, the same text may be harder to understand when heard than when read
(Chall 1983b; Dale and Chall 1995; Sticht, Beck, et al. 1974).
The SMOG formula G. Harry McLaughlin (1969) published his SMOG
formula in the belief that the word length and sentence length should be
multiplied rather than added. By counting the number of words of more than two
syllables (polysyllable count) in 30 sentences, he provides this simple formula:
SMOG grading = 3 + square root of polysyllable count.
McLaughlin validated his formula against the McCall-Crabbs passages. He used
a 100 percent correct-score criterion. As a result, his formula generally predicts
scores at least two grades higher than the Dale-Chall formula.
The FORCAST formula The Human Resources Research Organization studied
the reading requirements of military occupational specialties in the U.S. Army
(Caylor, Sticht, Fox, and Ford. 1973). In order to resolve professional questions
about using a formula for technical material read by adults, the authors firstundertook the creating of a readability formula that would be:
1.

Based on essential Army-job reading material.

2.

Adjusted for the young adult-male Army-recruit population.

3.

Simple and easy for standard clerical personnel to apply without special
training or equipment.

The researchers first selected seven high-density jobs and 12 passages that
recruits are required to understand to qualify for them. They graded the passages
with the modified Flesch formula, finding them to range from the 6th to the 13th
grade in difficulty. They also selected 15 text variables to study for a new
formula. They next tested the reading ability of 395 Army recruits, and thendivided them into two groups, one with a mean-grade reading level of 9.40 andanother 9.42.
They next tested the recruits with cloze tests made of the 12 passages. The 12
passages were then re-graded using the criterion of at least 50% of those subjectsof a certain grade level being obtaining a cloze score of at least 35%. Results
indicated that average subjects scored 35.1% on the text graded 9.1 and 33.5%
on the text graded 9.6.
They next intercorrelated the results of the reading tests with the results of thegraded cloze tests. Results showed usable correlations of .83 and .75 for the twogroups of readers. Among the 15 variables they examined, the number of onesyllable words in the passage correlated highest (.86) and was selected for use in
their new formula. Because they found that adding a sentence factor did not
improve the reliability of the formula, they left it out.

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The Principles of Readability

The resulting FORCAST formula is:
Grade level = 20 – ( N ÷ 10 )Where N = number of single-syllable words in a 150-word sample.
The new formula correlated r = 9.2 with the Flesch Reading Ease formula, 9.4
with the original Dale-Chall formula with, and r = .87 with the graded text
passages with. It is accurate from the 5th to the 12th grade.
They cross-validated the formula with a second study using another sample of
365 Army recruits at Ford Ord using another sample of reading passages scaled
from grade 7 to grade 12.7 using the FORCAST formula. The results of thisexperiment correlated r = .98 with the Flesch formula, .98 with Dale-Chall, and.77 with the graded military passages. These figures were judged appropriate forthe purpose of the formula.
Using the FORCAST formula, they tested the critical job-reading materials forreadability. The results show the percentage of materials in each occupation
written at the 9.9 grade level: Medical specialist, 24.4%; Light Weapons
Infantryman, 18.3%; Military Policeman, 15.1%; General Vehicle Repairman,
13.4%; Amorer/Unit Supply Specialist, 10.8%; Ground Control Radar
Repairman, 4.2%, and Personnel Specialist, 2.2%.
The study showed that materials for the different occupations all had texts above
the 9th grade. This suggested the need for new quality-control measures formaking materials more useful for the majority of personnel.
Fig. 13. Thomas Sticht. After participatingin the military studies which resulted in the
FORCAST readability formula, he went on
to become a worldwide authority in adultliteracy. He is shown here with UNESCO's
Mahatma Gandhi Medal he received in2003 for his contributions to that field. The
citation reads:"The UNESCO Mahatma Gandhi Bronze
Medal has been awarded to Dr. Thomas
Sticht (USA) in deep appreciation of his 25
years of service as a member of UNESCO's
International Jury for Literacy Prizes and inrecognition of his devotion to the cause of
adult literacy, especially for his efforts to'reach the unreached.'"In a follow-up study, Lydia Hooke and colleagues (1979) validated of the use of
the FORCAST formula on technical regulations for the Air Force. They also
found that four of seven writers of the regulations underestimated the grade levelof their materials by more than one grade.
In the main portion of the Hooke study, they administered cloze and reading tests
to 900 AF personnel to determine the comprehension of each regulation by theuser audience. Where there was no literacy gap (difficulty too high for thereader), they found that comprehension was adequate (at least 40% cloze score)in all cases. Where a literacy gap did exist, comprehension scores were below
the criterion of 40% in three of four cases.
The FORCAST formula is very unusual in that it does not use a sentence-length
measurement. This makes it a favorite, however, for use with short statements
and the text in Web sites, applications, and forms. The Department of the Air
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The Principles of Readability

Force (1977) authorized the use of this formula in an instruction for writing
understandable publications.
The following are two of the scaled passages taken from training materials andused in the occupational specialty study for the development and validation of
the FORCAST formula. Also shown are: 1. The scaled Reading Grade Level(RGL), the mean reading grade level of the subjects who scored 35% correctscores on the cloze tests; and 2. The scores of the FORCAST, the Flesch, and theoriginal Dale-Chall readability grade levels.
Passage 21
If you do not have a compass, you can find direction by other methods.
The North Star. North of the equator, the North Star shows you true
north. To find the North Star—
Look for the Big Dipper. The two stars at the end of the bowl are called
the “pointers.” In a straight line out from the pointers is the North Star(at about five times the distance between the pointers). The Big Dipper
rotates slowly around the North Star and does not always appear in thesame position.
You can also use the constellation Cassiopeia. This group of five brightstars is shaped like a lopsided M (or W, when it is low in the sky). The
North Star is straight out from the center star about the same distanceas from the Big Dipper. Cassiopeia also rotates slowly around the North
Star and is always almost opposite the Big Dipper.
Scaled RGL = 6. FORCAST = 8.6. Flesch = 7. Dale-Chall =7-8.
Passage 15
Adequate protection from the elements and environmental conditions
must be provided by means of proper storage facilities, preservation,
packaging, packing or a combination of any or all of these measures.
To adequately protect most items from the damaging effects of water orwater-vapors, adequate preservation must be provided. This is often
true even though the item is to be stored in a warehouse provided withmechanical means of controlling the temperature and humidity. Several
methods by which humidity is controlled are in use by the military
services. Use is also made of mechanically ventilating anddehumidifying selected sections of existing warehouses. Appropriate
consideration will be given to the preparation and care of items stored
under specific types of storage such as controlled humidity,
refrigerated, and heated. The amount and levels of preservation,
packaging, and packing will be governed by the specific method ofstorage plus the anticipated length of storage.
Scaled RGL = 11.4. FORCAST = 12.1. Flesch = 13-16. Dale-Chall =13-15.

The Army’s Automated Readability Index (ARI) For the U.S. Army, Smithand Senter (1967) created the Automated Readability Index, which used anelectric typewriter modified with three micro switches attached to cumulative
counters for words and sentences.
The ARI formula produces reading grade levels (GL):GL = 0.50 (words per sentence) + 4.71 (strokes per word) – 21.43.
Smith and Kincaid (1970) successfully validated the ARI on technical materials
in both manual and computer modes.
The Navy Readability Indexes (NRI) Kincaid, Fishburne, Rogers, andChissom (1975, Fishburne 1976) followed a trend by recalculating new versionsof older formulas and testing them for use on Navy materials. The first part ofthe experiment aimed at the recalculation of readability formulas. The second
Copyright © 2004 William H. DuBay

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The Principles of Readability

part of the study aimed at validating the effectiveness of the recalculated
formulas on Navy materials as measured by:•Comprehension scores on Navy training manuals•

Learning time, considered being an important measurement of
readability.

The first part of the study first determined the reading levels of 531 Navy
personnel using the comprehension section of the Gates-MacGinitie reading test.
At the same time, they tested their comprehension of 18 passages taken from
Navy training manuals. The results of those tests were used in calculating thegrade levels of the passages. They then used those passages to recalculate the
ARI, Flesch, and Fog Count formulas for Navy use, now called the Navy
Readability Indexes (NRIs). The recalculated grade-level (GL) formulas are:
ARI simplified:
GL = .4 (words per sentence) + 6 (strokes per word) – 27.4
Fog Count new:
GL = ((easy words + 3 (hard words)) ⁄ (sentences) ) – 3
2
Where:
easy words = number of number of 1 and 2-syllable words per 100
words
hard words = number of words of more than 2 syllables per 100 words
sentences = number of sentences per 100 words
Flesch Reading Ease formula simplified and converted to grade level (nowknown as the Flesch-Kincaid readability formula):
New:
GL = (.39 x ASL) + (11.8 x ASW) – 15.59
Simplified:
GL = ( .4 ASL ) + ( 12 ASW ) – 15
Where:ASL = average sentence length (the number of words divided by the
number of sentences).
ASW = average number of syllables per word (the total number
syllables in the sample divided by the number of words).
The second part of the study looked at the relationship between readability andlearning time. It monitored the progress of 200 Navy technical-training students
through four modules of their course for both comprehension and learning time.
The study was replicated with a secondary sample of 100 subjects performing onfour additional modules.
The results of the comprehension test showed the highest percentage of errors inboth the readers with the lowest reading grade levels and in the modules with thehighest grade-levels of readability.
In the same manner, the learning time systematically decreased with reading
ability and increased with the difficulty of the modules. The study confirms thatlearning time as well as reading ability are significant performance measures forpredicting readability.
The new Flesch-Kincaid formula was able to predict significant differences
between modules less than one grade level apart using both comprehension
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The Principles of Readability

scores and learning times. The U.S. Department of Defense (1978) authorized
this formula in new procedures for validating the readability of technicalmanuals for the Armed Services. The Internal Revenue Service, and the Social
Services Administration also issued similar directives.
Both Kern (1979) and Duffy (1985) urge the military to abandon use of theformulas. They note that writers in the military often find the task of simplifying
texts below the 10th grade “too difficult” and “not worth the trouble.”
Unfortunately, there are no practical alternatives to the skill hard work required
to create simple language. When large numbers of readers are involved, evensmall increases in comprehension pay off.
The Hull formula for technical writing At the 1979 STC conference, Leon C.
Hull (1979) argued that technical writing, with its increased use of difficult
words, needs a special kind of formula. While acknowledging that the
FORCAST and Kincaid formulas were developed precisely for that reason, helooked for a formula that does not use word length as a variable.
Basing his work on Bloomer (1959) and Bormuth (1969) as well as his own
experience as a technical writer, Hull claims that an increase in the number ofadjectives and adverbs before a noun lowers comprehension. His study indicatesthat the modifier load is almost as predictive as a syllable count, more causal,
and more helpful for rewriting.
Hull devised four cloze tests of each of five criterion passages from the Kincaid
study. The first test was the original passage. Each of the other tests increased
one of three indicators of modifier load by at least 50%: density of modifiers,
ambiguity of modifiers, and density of prepositions. The subjects were 107
science, engineering, and management students enrolled in a senior course intechnical and professional communication at Rensselaer Polytechnic Institute.
The mean cloze scores on the five unaltered passages correlated (r = ) 0.882 with
the Kincaid reading-grade levels assigned to these passages. This result justified
both the subject sampling and the use of the test results to produce a new
formula. The test results confirm the negative effect (r = -0.664) of modifier
density on comprehension. They also indicated that sentence length is a valid
indicator for technical material, perhaps better than word difficulty (contrary toprevious research).
Hull developed first formula with five variables, which accounts for (r2 = ) 68%
of passage difficulty. Like others before him, he found that the difficulty of usinga larger number of variables reduces the reliability of the formula and makes it
impractical. He created a another formula, shown here, that uses only sentence
length and the density of modifiers (called prenomial modifiers) and accounts for(r2 = ) 48% of passage difficulty. Though slightly less valid than the Kincaid
formula, it is as accurate as many other popular formulas:
Grade level = 0.49 (average sentence length)+ 0.29 (prenomial modifiers per 100 words) – 2.71
In the conclusion of his paper, Hull advises technical writers that using shorter
sentences reduces their complexity and makes them easier to read. He also
recommends eliminating strings of nouns, adjectives, and adverbs as modifiers.
Instead, writers should use prepositional phrases and place adjectives in thepredicate position (after the verb) rather than in the distributive position (before
the noun).
Degrees of Reading Power (DRP) In 1981, the College Entrance Examination
Board dropped its use of grade-level reading scores and adopted the Degrees of

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The Principles of Readability

Reading Power (DRP) system developed by Touchstone Applied Science
Associates (Koslin et al. 1987, Zeno et al. 1995).
The DRP uses the Bormuth Mean Cloze formula to predict scores on a 0 (easy)
to 100 (difficult) scale, which can be used for scoring both text readability andstudent reading skills. The popular children's book Charlotte’s Web has a DRP
value of 50. Likewise, students with DRP test scores of 50 (at the independentlevel) are capable of reading Charlotte’s Web and easier texts independently.
The Board also uses this system to provide readability reports on instructional
materials used by school systems.
Computerized writing aids Beginning in the 1980s, the first computer
programs appeared that not only contained the formulas but also other writing
aids. The Writer’s Workbench, developed at Bell Laboratories became the mostpopular of these (Macdonald, Frase, Gingrich, and Keenan 1982). It contains
several readability indexes, stylistic analysis, average lengths of words andsentences, spelling, punctuation, faulty phrases, percentages of passive verbs, areference on English usage, and many other features.
Kincaid, Aagard, O’Hara, and Cottrell (1981) developed CRES, a computerreadability editing system for the U.S. Navy. It contains a readability formulaand flags uncommon words, long sentences, and offers the writer alternatives.
Today, popular word processors such as Microsoft Word and Corel WordPerfectinclude a combination of spell checkers, grammar checkers, and readability
formulas to help in creating texts that are more readable. Note that the FleschKincaid Grade Level in Word’s Readability Statistics is defective in that it only
goes to the 12th grade.
Lexile Framework At the height of the controversy about the readability
formulas, the founders of MetaMetrics, Inc. (Stenner, Horabin, et al. 1988a)
published a new system for measuring readability, Lexile Framework, whichuses average sentence length and average word frequency found in the American
Heritage Intermediate Corpus (Carroll et al. 1971) to predict a score on a 0–2000 scale. The AHI corpus includes five million words from 1,045 publishedtitles to which students in grades three through nine are commonly exposed.
The cognitive theorists had claimed that different kinds of reading tests actually
measure different kinds of comprehension. The studies of the Lexile theorists(Stenner et al. 1988b, Stenner and Burdick 1997) indicate that comprehension isa one-dimensional ability that subsumes different types of comprehension (e.g.,literal or inferential) and other reader factors (e.g., prior knowledge and special
subject knowledge). You either understand a passage or you don’t.
The New Dale-Chall Readability Formula In Readability Revisited: The New
Dale-Call Readability Formula, Chall and Dale (1995) updated their list of
3,000 easy words and improved their original formula, then 47 years old. The
new formula was validated against a variety of criteria, including:•

32 passages tested by Bormuth (1971) on 4th to 12th-grade students.

•

36 passages tested by Miller and Coleman (1967) on 479 college
students.

•

80 passages tested by MacGinitie and Tretiak (1971) on college andgraduate students.

•

12 technical passages tested by Caylor et al. (1973) on 395 Air Forcetrainees.

The new formula was also cross-validated with:•
Copyright © 2004 William H. DuBay

The Gates-MacGinitie Reading Test

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The Principles of Readability•

The Diagnostic Assessments of Reading and Trial Teaching Strategies(DARTTS)•The National Assessment of Reading Progress•

The Spache Formula•

The Fry Graph•

Average judgments of teachers on the reading level of 50 passages ofliterature
The new formula correlates .92 with the Bormuth Mean Cloze
Scores, making it the most valid of the popular formulas.

Fig 14. Jeanne S.
Chall. She createdthe Harvard Reading
Lab and directed it for
20 years.

At the time of writing this, the new Dale-Chall formula is not yet
available on the Internet. It was once available in a computer
program, “Readability Master,” but is hard to find. You can easily
apply the formula manually, however, using the instructions,worksheet, word list, and tables provided in the book. The bookalso has several chapters reviewing readability research, the usesof the formulas, the importance of vocabulary, the readability
controversies, and a chapter on writing readable texts.
The following are two of the sample passages in the book, with thedifficult words not found on their new word list underlined (pp.
135-140). The right-hand column gives a few readability statistics,
the New Dale-Chall mean cloze score, and reading grade level.

Grades 5-6
Eskimos of Alaska’s Arctic north coast
have hunted whales for centuries.
Survival has depended on killing the 80foot-long bowhead whales that swim fromthe Bering Sea to the ice-clogged
Beaufort Sea each Spring. The Eskimos’entire way of life has been centeredaround the hunt.

Readability Data
Number of Words in Sample

100

Number of Whole Sentences

6

Number of Unfamiliar Words

11

Cloze Score

42

Reading Level

5-6

But now that way of life is being
threatened by America’s need for oil, say
many Eskimos who hunt the whales.
Huge amounts of oil may be beneath the
Beaufort Sea. And oil companies want tobegin drilling this spring.
However, many Eskimos say severe
storms and ice conditions make drilling
dangerous…
From My Weekly Reader, Edition 6

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The Principles of Readability

Grades 9-10
The controversy over the laser-armed
satellite boils down to two related
questions: Will it be technically effective?
And should the United States make amassive effort to deploy it?
To its backers, the laser seems theperfect weapon. Traveling in a straightline at 186,000 miles per second, a laser
beam is tens of thousands of times as
fast as any bullet or rocket. It could strike
its target with a power of many watts per
square inch. The resulting heat, combinedwith a mechanical shock wave created byrecoil as surface layers were blasted
away, could quickly melt…

Readability Data
Number of Words in Sample

100

Number of Whole Sentences

5

Number of Unfamiliar Words

23

Cloze Score

28

Reading Level

9-10

From Discover

ATOS readability formula for books Researchers at School Renaissance
Institute (1999, 2000, Paul 2003) and Touchstone Applied Science Associates
produced the Advantage-TASA Open Standard (ATOS) Readability Formula for
Books. Their goal was to create an “open” formula that would be available to theeducational community free of charge, that would be easy to use, and that could
be used with any nationally normed reading tests.
The project was perhaps the most extensive study of readability ever conducted.
Formula developers used 650 norm-referenced reading tests, 474 million words
representing all the text of 28,000 K–12 books read by real students with many
published in the previous five years, an expanded vocabulary list, and the reader
records of more than 30,000 students who read and tested on 950,000 actual
books.
The readability formula was part of a computerized system to help teachers
conduct a program of guided independent reading to maximize learning gains.
Noting the differences in difficulty between samples and entire books, thedevelopers claim this is the first readability formula based on whole books, notjust samples.
They found that the combination of three variables gives the best account of text
difficulty: words per sentence (r2 = .897), the average grade-level of words (r2 =.891), and characters per word (r2 = .839). The formula produces grade-level
scores, as they are easier for teachers to understand and use.
The formula developers paid special attention to the Zone of Proximal
Development (ZPD) proposed by Vygotsky (1978), the level of optimaldifficulty that produces the most learning gain. They found that, for independent
reading below the 4th grade, maximum learning gain requires at least 85%
comprehension. Advanced readers need a 92% score on reading quizzes. Those
who exceed that percentage should be given material that is more challenging.
Other results of the studies indicate that:

Copyright © 2004 William H. DuBay•

Maximum learning gain requires careful matching of book readability
and reading skill.

•

The amount of time spent reading correlates highly with gains inreading skill.

•

Book length can be a good indication of readability.

•

Feedback and teacher interaction are the most important factors inaccelerated reading growth.

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The Principles of Readability

Formula Applications
Many researchers outside the field of reading have recognized the value of theformulas. Edward Fry (1986) points out that articles on the readability formulas
are among the most frequently cited articles of all types of educational research.
The applications give researchers an objective means of controlling the difficultyof passages in their experiments.
The following is a sample of readability studies that used formulas: political
literature (Zingman 1977), corporate annual reports (Courtis 1987), customer
service manuals (Squires and Ross 1990) drivers’ manuals (Stahl and Henk
1995), dental health information (Alexander 2000), palliative-care information(Payne et al. 2000), research consent forms (Hochhauser 2002; Mathew 2002;
Paasche-Orlow et al. 2003), informed consent forms (Williams et al. 2003)
online health information (Oermann and Wilson 2000), lead-poison brochures(Endres et al. 2002) online privacy notices (Graber et al. 2002) medical journals(Weeks and Wallace 2002), environmental health information (Harvey andFleming 2003) and mental-health information (King et al. 2003).
Court actions and legislation Fry (1989a) points out that the validity of theformulas has been challenged in court and found suitable for legal purposes. The
courts increasingly rely on readability formulas to show the readability of texts inprotecting the rights of citizens to clear information. Court cases and legislation
involving government documents and correspondence, criminal rights, productlabeling, private contracts, insurance policies, ballot measures, warranties, andwarnings are some of the legal applications of the formulas.
In 1984, Joseph David of New York was upset by his inability to understand a
letter of denial he received in response to his appeal for Medicare benefits. Legal
Services went to court in behalf of David and other elderly recipients ofMedicare in New York. They pointed out that 48% of the population over 65
had less than a 9th-grade education. Edward Fry testified in court that the denial
letter was written at the 16th-grade level. As a result, the judge ordered theSecretary Heckler of the U.S. Department of Health and Social Services to take“prompt action” to improve the readability of Medicare communications (Davidvs. Heckler 1984).
A number of federal laws require plain language such as the Truth in Lending
Act, the Civil Rights Act of 1964, and the Electronic Funds Transfer Act. In June
1998, President Clinton directed all federal agencies to issue all documents andregulations in plain language.
Beginning in 1975, a number of states passed plain-language laws covering suchcommon documents as bank loans, insurance policies, rental agreements, andproperty-purchase contracts. These laws often state that if a written
communication fails the readability requirement, the offended party may sue andcollect damages. Such failures have resulted in court judgments.
States such as California also require plain language in all agency documents,
including “any contract, form, license, announcement, regulation, manual,
memorandum, or any other written communication that is necessary to carry out
the agency's responsibilities under the law” (Section 6215 of the California
Government Code). California defines plain language as “written or displayed sothat the meaning of regulations will be easily understood by those persons
directly affected by them” (Section 11349 of the Administrative Code).
Textbook publishers After 80 years, textbook publishers consider the gradelevel of textbooks as more important than cost, the choice of personnel, or thephysical features of books. All of them use word-frequency lists. Eighty-nine
percent of them use readability formulas in evaluating the grade-levels of texts,
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The Principles of Readability

along with other methods of testing. Widely read children’s publications such as
My Weekly Reader and magazines published by National Geographic forchildren of different ages have used the formulas along with field-testing andother methods (Chall and Conard 1991).

Using the Formulas
Formula discrepancies The discrepancy between the scores of different
formulas has long been perplexing. For example, the scores for the following
four paragraphs are:Original Dale-Chall grade level: 11-12
Flesch grade level: 8.9
FORCAST grade level: 10.9
SMOG grade level: 11.7
Fog grade level: 12.3
Critics have often cited such discrepancies as indications of the lack of precisionof the formulas. Kern (1979) argued that the discrepancies among the Kincaid
and Caylor formulas deprive them of usefulness, and that the military should
discard them. What Kern ignores in his review are the correlations of theformulas with comprehension tests. What is important is not how the formulas
agree or disagree on a particular text, but their degree of consistency inpredicting difficulty over a range of graded texts.
The most obvious causes of the discrepancies are the different variables used bydifferent formulas and the different criterion scores used in their development.
The formulas—like reading tests—simply do not have a common zero point(Klare 1982). The criterion score is the required level of comprehension
indicating reading success as indicated by the percentage of correct answers on areading test. For example, a formula can predict the level of reading skill
required to answer correctly 75 % of the questions on a reading test based on acriterion passage.
The FORCAST and Dale-Chall formula use a 50% criterion score as measured
by multiple-choice tests. The Flesch formula use a 75% score, Gunning Fog
formula, a 90% score, and the McLaughlin SMOG formula a 100% score. The
formulas developed with the higher criterion scores tend to predict higher scores,
while those the highest validity correlations (e.g., Dale-Chall and Flesch) tend to
predict lower scores.
The different methods used by different computer programs to count sentences,words, and syllables can also cause discrepancies—even though they use thesame formula. Finally, the range of scores provided by different formulas remind
us that they are not perfect predictors. They provide probability statements orrather estimates of difficulty.
The problem of optimal difficulty Different uses of a text require different
levels of difficulty. As we have seen, Bormuth (1969) indicated the 35% cloze
score was the point of optimum learning gain (see Table 7 above) for assistedclassroom reading.
Vygotsky (1978) supported Bormuth’s findings that optimal difficulty should beslightly above their current level of development and not below. Using books
that are at the reader’s present level or below may increase fluency and rate, but
not in the way of comprehension.
For this reason, reading experts advise that materials intended for assisted
reading when an instructor is available should be somewhat harder than thereaders’ tested reading level. Materials for the general public, however, such as
medicine inserts, instructions for filing tax forms, instructions for using
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appliances, and health information should be as easy as possible to convey(Chall and Dale 1995).
Paul (2003) found that independent reading requires at least an 85%
comprehension on multiple-choice reading quizzes for readers below the 4th
grade and 92% for advanced readers. He also recommends that advanced
students who score better than 92% correct on quizzes should be given material
that is more challenging.
The formulas and usability testing Redish (2000) and Schriver (1991, 2000),
promote the need for reading protocols and usability testing as an alternative tothe formulas. They feel that usability testing eliminates the need for readability
testing. They fail to state, however, how to match the reading ability of subjects
with that of the target audience.
Dumas and Redish (1999), in their work on usability testing, hardly mention
reading comprehension. They have us assume that, if test subjects correctly
perform a task, they have correctly understood the instructions. When problems
arise, however, it is difficult to locate the source of the difficulty.
In both usability testing and reading protocols, some subjects are more skilledthan others in articulating the problems they encounter. Do problems come fromthe text or from some other source? If they are located in the text, do they comefrom the design, style, organization, coherence, or content? We are often left
with guesswork and trial-and-error cycles of revision and testing.
As experience has taught us, this gets expensive. In developing a text, it makes
as little sense to neglect the readability of a document as it does to neglect its
punctuation, grammar, coherence, or organization. Readability is not a trivial
issue. If an audience cannot read a text, they cannot understand it. If they cannot
understand it, they cannot use it to complete a task.
Schriver (1997) and Hackos and Redish (1998) correctly emphasize theimportance of testing and of frequent consultation with members of the targeted
audience before, during, and after developing a document. Assessing both thereading ability of the audience and the readability of the text will greatly
facilitate this process.

Conclusion
Today, the readability formulas are more popular than ever. There arereadability formulas for Spanish, French, German, Dutch, Swedish, Russian,Hebrew, Hindi, Chinese, Vietnamese, and Korean (Rabin 1988).
The formulas have survived 80 years of intensive application, investigation, andcontroversy, with both their credentials and limitations remaining intact. The
national surveys on adult literacy have re-defined our audience for us. Any
approach to effective communication that ignores these important lessons cannot
claim to be scientific. If we walk away from this research, others will one day
rediscover it and apply it to our work as technical communicators.
The variables used in the readability formulas show us the skeleton of a text. It is
up to us to flesh out that skeleton with tone, content, organization, coherence,and design. Gretchen Hargis of IBM (2000) states that readability research has
made us very aware of what we “write at the level of words and sentences.” She
writes:Technical writers have accepted the limited benefit that thesemeasurements offer in giving a rough sense of the difficulty ofmaterial.

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We have also assimilated readability as an aspect of the quality of
information through its pervasiveness in areas such as task orientation,completeness, clarity, style, and visual effectiveness. We have put into
practice, through user-centered design, ways to stay focused on theneeds of our audience and their problems in using the information or
assistance that we provide with computer products.
The research on literacy has made us aware of the limited reading abilities ofmany in our audience. The research on readability has made us aware of themany factors affecting their success in reading. The readability formulas, whenused properly, help us increase the chances of that success.

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Cloze Answers .
The potential for two-way communication is very strong on the Web. As aresult, many companies are focused on the Web’s marketing potential.
From a marketing point of view, today’s virtual worlds can attract the
curious Web explorers, and interactive database engines can measure andtrack a visitor’s every response.

Biosketch
William H. DuBay worked as a technical communicator in several California
firms for 15 years. He is a Senior Member of Orange County chapter of theSociety for Technical Communications and contributes frequently to its
publications. He serves on the Advisory Board of the Technical and Professional
Communication Certificate Program of California State University, Long Beach.
He also belongs to the Plain Language Association International. Currently, heworks as a readability consultant. His email address is bdubay@sbcglobal.net
The author wishes to thank George Klare for his guidance and help in thedevelopment of this article.

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