Presented at the American Association for Public Opinion Research, Norfolk, Virginia, May 1997.

1

PROCEDURES FOR REDUCING MEASUREMENT ERROR IN ESTABLISHMENT SURVEYS

Karen Goldenberg, Bureau of Labor Statistics; Kerry Levin, Tracey Hagerty, Ted Shen, David Cantor, Westat

Karen L. Goldenberg, Bureau of Labor Statistics, 2 Massachusetts Ave. N.E., Room 4985, Washington DC 20212

KEY WORDS: Pretesting, Establishment Survey,

Screening, Questionnaire Design1

1. Measurement Error in Establishment Surveys

In this paper we discuss two sets of procedures

designed to reduce measurement error during two

separate phases of an establishment survey. The first

set is concerned with questionnaire design and pretesting. The second set focuses on steps to identify correct

respondents and to contact them for the survey.

Survey measurement error is generally defined as

the difference between the answer presented in a

survey situation and the true value that applies to that

answer. It can originate with the respondent, the mode

of data collection, the data collection instrument, and

the interviewer (Groves, 1989). In establishment

surveys, the establishment’s recordkeeping or

information system may also contribute (Edwards and

Cantor, 1991).

The term establishment survey refers to data collection efforts designed to study the characteristics or

attributes of organizational entities such as businesses,

farms, and institutions (Cox and Chinnappa, 1995).

Information sought in such surveys is often recordbased administrative and financial data. Therefore

researchers usually designate as the respondent a

person with specialized knowledge about the survey

topic. Locating the correct respondentone who

understands the survey questions and has knowledge of

and access to relevant records and other

informationbecomes critically important (Edwards

and Cantor, 1991).

Once the designated respondent has been identified

and has agreed to participate in the survey, other

sources of error, most not unique to establishment surveys, come into play. Common sources of error include

whether the correct variables are being measured, the

validity of those measures, whether the instrument

items mean to the respondent what was intended by the

researcher, and completeness and adequacy of the

response options. Of course, these sources of error are

interdependent. As an example, a poorly-worded question can cause a respondent to provide an incorrect,

incomplete, or inadequate answer, and at the same

time cause the interviewer to deviate from the

structured interview script to assist the respondent. In

 

1

 Views expressed in this paper are those of the authors and do not

reflect policy of the Bureau of Labor Statistics or Westat.

establishment surveys, if the information system cannot

easily satisfy an information request, a respondent may

provide an incorrect answeror no answer.

2. Background: The Business Births Pilot Study

This paper describes research performed during the

Business Births Pilot Study (BBPS), conducted by

Westat, Inc. under contract to the U.S. Bureau of Labor

Statistics (BLS). The BBPS was created within the

context of BLS’ Current Employment Statistics (CES)

Survey, the source of monthly data on changes in U.S.

nonfarm payroll employment. BLS conducts the CES

each month with a sample of approximately 400,000

business establishments. While the sample is large, it

does not currently include businesses in the early

months of their existence. The BBPS was designed to

assess whether we could identify these new businesses,

called business births, by means of a short telephone

interview with the owner or manager of the business,

and collect employment data from them.

2.1 Business Births

For purposes of this research, a business birth is an

establishment, with employees, which formerly had no

chance of being selected in the CES survey. Selection

into the CES sample is through an employer’s Unemployment Insurance (UI) account. The UI program

functions separately within each state, and employers

open UI accounts in all states where they have

operations. Most states require employers to register

for an account within either 90 or 180 days of

becoming liable for UI taxes in that state, although

compliance with this requirement is not universal.

While many new UI accounts go to new businesses,

other reasons for obtaining them include changes in

ownership, mergers or acquisitions, name changes, and

incorporations. In general, a new UI account assigned

to a continuing economic venture (such as an

ownership change) had a prior probability of selection

into CES and is not considered a business birth.

However, an established business may become a birth

by opening a business unit in a state where it did not

previously have an account. Similarly, an organization

that moves from one state into another becomes a birth

in the new stateand a death in the old one.

2.2 Data collection

The BBPS, a telephone survey with monthly data

collection, took place between July and November,

1996. Each month, BLS selected a sample of new UI
2

accounts and transferred that sample to Westat. Westat

interviewers screened the sample units, sent advance

letters, and then contacted the designated respondents

to conduct a Computer Assisted Telephone Interview

(CATI).

2.3 Sample

BLS asked State UI offices for files of new account

recipients. Over the course of pretesting and data collection, we used UI accounts from 6 states. We selected

a sample of 250 cases for July data collection and 500

in each subsequent month, with approximately equal

numbers of cases per state. Files were stratified on the

basis of employer size (less than 50 employees and

unknown employment, or 50+ employees), with

random selection within strata.

3. Development of the BBPS Questionnaire

The BBPS questionnaire was designed for telephone administration to business establishments. The

purpose of the questionnaire was to differentiate

between business births and continuing economic units

among employers that had recently received new UI

accounts, and, for the birth units, to measure employment. In designing the questionnaire, we focused on

the reason for obtaining a new UI account. When that

reason indicated a birth, we looked at when the

business began, employment at the time of the birth,

and current employment. "Current employment" later

evolved to employment under the CES concept, which

is the number of paid employees for the pay period

including the 12th of the month. We anticipated that

the majority of new UI accounts would go to small

businesses, and that the respondent would usually be

the establishment’s owner or manager.

The discussion below presents our efforts to reduce

measurement error in this study by (1) conducting cognitive research to assess respondent understanding of

key concepts; (2) iterative paper and pencil pretesting

to identify and resolve question wording problems; and

(3) behavior coding of the first wave of production

interviewing to evaluate respondent behavior vis-à-vis

the survey items. Questionnaire development and testing took place in 1995 and early 1996.

3.1 Initial design and testing: Cognitive Research

A BLS research team began questionnaire development by attempting to resolve what we meant by

“business birth.” It was unclear at the outset whether

respondents would be familiar with unemployment

insurance accounts and whether they could provide the

reason their business had received a new one. There

were "mechanical" issues to be resolved, such as how

to refer to the business if we were speaking to someone

other than the business owner, as well as whether to

use open-ended or forced-choice questions on key

items.

The first phase of questionnaire testing consisted of

a series of cognitive interviews with recent recipients

of new UI accounts. Examples of key outcomes from

these interviews include: (a) respondents don't necessarily understand what a UI account is, but can still

explain why they got a new one; (b) respondents

thought in terms of opening a “new” business whether

they started one from scratch or purchased it from

someone else; and (c) our proposed list of reasons for

new UI accounts had missed a very important reason,

“hiring employees for the first time.” (See Goldenberg,

1996, for a more detailed discussion of the initial

research.)

3.2 The CATI QuestionnaireIterative Pretesting

Pretesting is a critical part of any survey. A new

study should have a minimum of two pretests, one that

tests initial wording and one that serves as a dress

rehearsal for the study as a whole (Converse and

Presser, 1986). A second pretest is also important

because replacements for questions that didn’t work in

the first pretest need to be tested; sometimes the new

questions are no better or could even be worse than the

originals (Sudman et al., 1996).

We conducted three paper and pencil pretests for

the BBPS. While we did not plan a priori to conduct

three separate pretests, we were aware of the likelihood

of changes after the initial pretest, and the need to

retest the questionnaire following those changes. We

decided on a third pretest because we continued to

modify the questionnaire and needed to test those

changes. We document the BBPS pretest experience

here.

3.2.1 Pretest I. The first pretest took place June

3-4, 1996, and resulted in 26 completed interviews out

of 40 sample cases. Two experienced Westat interviewers conducted all pretest interviews, which

averaged 7 minutes. To evaluate the pretest data, project staff monitored the interviews, and held an informal debriefing with the interviewers. We also compiled

a question-by-question summary of every situation

where the interviewer was not able to immediately fit

the response into a predefined category, or where our

subjective assessment showed that the respondent

experienced some difficulty with the question. We

describe specific pretest results below.

Introduction. The survey introduction attempted to

interest the respondent in the study and described the

sampled business unit. In addition, it included a reference to the advance letter (see section 4.3), a confiden-
3

tiality statement, and a request for the respondent’s

permission to tape the interview. If the respondent did

not remember seeing the advance letter, the interviewer

read it aloud.

Changes: During the introduction, the research

team observed signs of respondent impatience, so we

shortened it slightly. For respondents who didn’t recall

receiving the letter, we allowed the interviewer to fax it

instead of reading it aloud. We changed the reference

to the specific business unit from its complete address

to “on [NAME OF] street,” and eliminated a second use

of that address.

Reason for new UI account. The questionnaire began

with an open-ended question on reasons for the new UI

account. All 26 respondents answered the unstructured

question. Overall, 18 of the 26 respondents (69%) gave

what we considered legitimate reasons, all of which

appeared on our predefined list. Eight of the 18 were

reasons related to having employees and needing a

vehicle for paying taxes, while the remaining 10 cited

new businesses, ownership changes, incorporations, or

other valid reasons. The remaining 8 respondents

(31%) referred to legal requirements or processes or

implied that they were merely following directions: “It’s

the law,” “my accountant said so,” “we’re supposed to.”

We followed the unstructured question with a list of

reasons why a business might obtain a new UI account.

The interviewers read each item, and asked

respondents to answer “yes” or “no” to each one. The

project staff compiled a summary of all responses to

the unstructured “reason” question and all individual

reasons for which the respondents said “yes.” Four

respondents said yes to only one reason. Sixteen

respondents gave two or more internally consistent

reasons (e.g., started new business and hired

employees), while the remaining six respondents gave

inconsistent reasons (e.g., purchased a business and

opened a new business). These inconsistencies had

potentially serious consequences, because the

respondent could be directed to the wrong section of

the questionnaire.

Changes: First, we eliminated the unstructured

question, on the grounds that it did not add new information. Second, we acknowledged that respondents

had an even broader definition of “new business” than

we had previously recognized. We dealt with this

situation by asking two questions, one specifically

about starting a new business or franchise, and the

other about the purchase of a business from someone

else. If the respondent started a new business, and did

not purchase it from another owner, we treated it as a

newly-created business. If the respondent bought a

business, we asked a series of questions about

purchase/ownership change. The interviewer asked

additional questions about the reason for a new account

only if the new UI account was for reasons other than

opening or purchasing a business.

Third, we modified the list of reasons based on

responses received in the pretest. We then structured

the list so that the more inclusive reasons came first,

and instructed the interviewer to read each item until

the respondent answered affirmatively. For example,

“incorporation” preceded “name change.”

Start date and initial employment. Comprehension

was not an issue in these questions, but obtaining

accurate information was. In some cases the

respondent did not know the answers but could obtain

them from another source. In other cases respondents

estimated the number of employees.

Changes: We trained interviewers to encourage

respondents to check records if possible. In some cases

interviewers offered to give respondents time to obtain

the data and call back, and to accept an estimate only

as a last resort. If the respondent had no idea, we told

the interviewer to obtain the name and telephone

number for another contact and to terminate the

interview.

Current employment. We attempted to obtain employment information for the pay period containing the

12th of the current month, or, if that pay period was

not yet over, current employment. Nearly a fifth of the

pretest respondents (5 of the 26) had difficulty with

questions relating the current pay period to the pay

period containing the 12th of the month. The pay

period of the 12th is an important economic concept

for BLS surveys, and its use relies on what Griffiths

and Linacre (1995) call the traditional assumption that

business surveys are straightforward, with data derived

directly from business records by respondents who

have a good understanding of economic concepts.

They note that this is not always the case, and that “the

economic concepts traditionally used in business

surveys are not easily understood by respondents”

(p. 675). The pretest data support this assertion. To

assist in locating the pay period, the questionnaire

contained the suggestion that respondents refer to a

calendar. The interviewers were fairly successful in

using it to help “walk” the respondent through the

questions. However, the price was considerable

deviation from the script and therefore significant

interviewer variance.

Changes: We moved the suggestion that the respondent refer to a calendar to the beginning of the series

on pay period questions. We addressed most of the pay

period of the 12th problems with interviewer training.
4

Repetitive wording. Respondents whose businesses

have employees were asked for the number of

employees during the pay period when those employees

first reported to work. The question is deliberately

long. so as to incorporate cues telling respondents

which employees to include in the count:

The number of employees in a company includes full and part time

workers, owners of incorporated businesses, temporary workers,

managers, executives, corporate officers, office and clerical

workers, and all other paid employees who are covered by

unemployment insurance. For the first pay period that [COMPANY

NAME] had employees, what was the number of paid employees?

No more than 3 questions later, we asked a nearly

identical question about employment for the pay period

of the 12th. The excessive wordiness was uncomfortable to interviewers and seemed to annoy respondents.

We were concerned that interviewers might not read

the questions as worded, so we rewrote the second

question in the series.

3.2.2 Pretests II and III. Both the second and third

pretests resulted in some changes to the questionnaire,

although there were fewer of them and they were less

extensive than the changes from Pretest I. Since some

of the same issues appeared in both pretests, we discuss

them together.

We conducted Pretest II on June 11 with a sample

of 30 cases, randomly selected from the same prescreened file as the first pretest. The sample yielded 20

completed interviews. Like Pretest I, the second

pretest identified new issues, questions, and problems.

Pretest III took place June 18-20, using an againrevised questionnaire and an initial sample of 102

cases. It yielded 62 completed interviews, with 9

refusals.

Most of the changes we made in the questionnaire

following Pretest I appeared to be successful. Modifications to the questions about reasons for a new UI

account made the overall interview smoother, and the

average length of interview dropped from roughly 7

minutes to about 5 minutes. At the same time, this

pretest revealed a few other problems, both with the

questionnaire and more broadly with the survey

procedures and datafiles.

Reason for UI account. As noted above, if the respondent did not open a new business or purchase one from

another owner, the interviewer read a list of possible

reasons. Several times the respondent interrupted

during the introduction to the question and revealed

the reason right away, but the interviewer still had to

read all of the items on the list up to and including the

appropriate reason. The interruptions signified that

the question still required work.

In addition, during the course of the interview three

respondents revealed different reasons for their new UI

registrations than the ones offered during their first

response, and at least one such change affected the

outcome variable (nonbirth instead of birth). The

presence of these “informal conversations” indicated that

the interviewers were still forced to deviate from the

script in order to obtain appropriate responses.

Changes: We continued to shorten the questions,

giving the interviewers fewer words to ask. To reduce

the likelihood of interruptions in the stem of the “other

reasons” question, we changed it to a single sentence,

which seemed to alleviate the problem of respondent
5

interruptions. However, in Pretest III there were still 6

cases where informal comments during the interview

contradicted the original reason. The interviewers were

able to go back and proceed through the correct questionnaire path. Since this was a recurring situation, we

decided to train interviewers to backtrack if necessary,

and made sure that the CATI operation would permit

it.

Who is an employee? During the cognitive research

we addressed the issue of whether owners of

incorporated businesses should be counted as

employees, and determined that they should be counted

only if they received pay. Owners of unincorporated

businesses are not employees and should not be

counted. During Pretest II the owner of an

unincorporated business included himself in the

employee count. Since we already asked whether the

business was incorporated, we added a reminder that

the total does not include owners to be used when we

interview at unincorporated businesses.

Current pay period/pay period of the 12th. Eight

respondents from Pretest II (40%) and eight from Pretest III (13%) had difficulty with these questions. After

Pretest II, we modified the question about current pay

period start date to ask about the day the current pay

period began, instead of the first date of the current

pay period. Given the much lower rate of difficulty in

Pretest III, the new wording appears to have helped.

Replacing UI accounts. The question about replacing

an existing account worked well as long as it was a

recent account. In the case of a renewed liability,

however, there was some confusion about whether an

account still existed. We changed the word “existing” to

active.

3.3 Behavior Coding

Behavior coding has been used since the 1970s to

evaluate interviewer behavior, and more recently to

evaluate questions for pretests (Oksenberg et al., 1991).

Most behavior coding schemes monitor whether the

interviewer asks the question exactly, modifies it

without changing the meaning, or modifies it significantly. These schemes also note whether the

respondent seems to give an adequate answer, asks for

clarification, expresses confusion or impatience,

interrupts the interviewer with an answer, doesn’t

know, or refuses to answer. If interviewers have

difficulty asking questions exactly as worded, the

problem can be either with interviewer performance or

with a question that is difficult to ask. If respondents

interrupt the interviewer the question is probably too

long or poorly worded (as we saw in the first and

second pretests). When respondents ask for

clarification or express confusion, again the question is

problematic. Measurement error results unless these

difficulties are addressed and corrected.

We used behavior coding to evaluate the questionnaire. Two coders analyzed tapes made of 91 of the

150 completed interviews from initial CATI administration period (July). In order to ensure that the two

coders were coding interviews in the same way, 39 of

the 91 interviews were coded twice—once by each coder.2

The analysis that follows is based on the 91 taped

interviews and the coding done by the first coder for

that interview. The coders selected interviews to code

at random, so this should not skew results.

Throughout the pretests, the pay period including

the 12th was a persistent “problem area” in the questionnaire. The behavior coding suggested that the pay

period questions required immediate attention. While

most respondents were able to supply an adequate

answer to these questions, behavior coding pointed to a

high level of confusion. Eighty percent of respondents

needed clarification from the interviewers about the

day the current pay period began, and 75 percent were

confused about the relationship between the current

pay period and the pay period containing the 12th of

the month. We resolved the issue by letting the CATI

system take over. We kept the question about the day

the current pay period began and we added a question

about the day the current pay period ended. In

conjunction with the information from a question about

how often the firm paid its employees, CATI was programmed to determine whether the current pay period

ended before, coincided with, or began after the pay

period of the 12th. The CATI system then displayed

the appropriate question and asked for the number of

employees.

There was relatively little evidence of difficulty

with other areas of the questionnaire. However, the

questionnaire has many complex routing skips, and

several of the questions were only asked a handful of

times, or were not asked in the coded interviews.

4. Identifying and Contacting Respondents

A factor which contributes to a decrease in measurement error and an increase in response rates is

identifying a respondent who is knowledgeable about

the survey content area. In this study the focus was on

payroll and employment, so the logical respondent was

the individual responsible for a business’ payroll.

 

2

 Subsequent review of these 39 interviews revealed that there were

systematic differences between the two, particularly in the area of

interviewer behavior. A third coder recoded some of the same interviews, with a higher percentage of agreement than between the first two

coders. See Levin et al., 1996, Appendix A, for a detailed discussion.
6

Results from other surveys confirm that identifying

the most knowledgeable person by name is important

to increasing response rates. Paxson et al. (1995)

examined the effect of sending a mail questionnaire to

a named respondent. Naming the respondent and

making repeated contacts produced some of the highest

response rates in the 26 surveys they reviewed.

However, Moore and Baxter (1993) used contact names

and other Total Design Method (TDM) procedures in a

mail survey of businesses, and generally found no

increase in response as a result. They did find a name

helpful, however, among some classifications of small

businesses. This is relevant to the present research, as

95% of the establishments are small businesses.

4.1 Preliminary Screening

BLS and Westat have both found prescreening to be

a useful technique for identifying the most appropriate

respondent in an establishment survey. Prescreening

consists of an initial telephone call to the establishment

at the outset of the project. During this phone call, the

person who answers the call is asked to verify information about the establishment and to provide the

name of the person most knowledgeable about the

content of the survey. Even if the screening respondent

does not supply the correct name, the named

respondent is a starting contact within the business.

In the BBPS, Westat began with the business

names and telephone numbers provided by BLS.

Interviewers called the sampled establishments and

identified themselves as calling from Westat on behalf

of the Bureau of Labor Statistics. They explained that

the purpose of the call was to confirm the business

address for a study that was to be conducted in the next

few weeks. The interviewers verified the business’s

mailing address and telephone number, obtained a fax

number if one was available, and asked for the name of

the person in the company who was responsible for

payroll.

The majority of businesses contacted during screening were small, with fewer than 10 employees. Consequently, the individual who answered the telephone

was often the owner, head of payroll, or office manager

and could also be the survey respondent. Interviewers

had to be prepared to answer questions regarding the

upcoming survey.

4.2 Screening Results

The primary purpose of screening is to identify a

respondent. However, contacting establishments also

allowed us to refine and update the sample file.

Identifying correct respondents. Out of a total of

2,248 establishments in our initial sample, we were

able to designate a respondent 78% of the time. We

sent advance letters directly to these respondents. Over

the five months of data collection, and for the

responding sample units, 92 percent of the completed

interviews were with named contacts identified during

screening. In the remaining cases, the named

respondent referred interviewers to another contact

(e.g., payroll processing firm, accountant); 45% of

these were at different locations from the original

respondent.

Identifying closed businesses. Screening allowed

us to identify firms that had gone out of business or

had otherwise stopped operation (3 percent of the

original sample). We excluded these units from the

sample file.

Eliminating file errors. Screening also allowed us

to eliminate a small number of cases that should never

have been included in the sample file. About 2% of the

file contained duplicate records, businesses that did not

have a location in the state being sampled, or

employers who had hired domestic help and were not

eligible.

Updating the file. Overall, about 12% of the sample

could not be located. Including cases that were out of

business or never answered the phone, over 17% of the

sample cases were ineligible for further interviewing.

4.3 Advance Letters

The literature strongly supports the use of letters to

respondents in advance of the actual survey. The

research evidence shows that they ultimately lead to

higher response rates, because they enhance the legitimacy of the study and help to prepare the respondent

for the survey. Therefore, once we identified the appropriate respondent, we sent a letter addressed to that

person by name. The letter was signed by the Commissioner of Labor Statistics. It described the survey, and

encouraged the respondent to participate.

During eligibility screening, we asked respondents

for a fax number. Across all months of data collection,

approximately 70% of respondents provided them. If

we had a fax number, we faxed the advance letter;

otherwise, we mailed it.

We included a question about the letter in the survey introduction, and most respondents recalled receiving it. Mail or fax made little difference: 75 percent of

the fax recipients, and 77 percent of the mail

recipients, recalled the letter. However, the time

between receipt of the letter and contact with the

interviewer by telephone was no longer than one week.

Did the advance letter have an effect on survey

response? We did not test this question

experimentally, but we think it must have. Response

rates for the study ranged from 72 percent to 87

percent each month, with an overall rate of 85 percent.
7

Rate calculations are based on eligible respondents

located during screening.

5. Summary

Overall, we have discussed a number of procedures

used to reduce measurement error. First, initial cognitive pretesting led to substantive rethinking of concepts being measured. Second, the questionnaire was

subjected to several rounds of pretesting, and the same

conceptual issues emerged. Changes made during

pretesting were tested iteratively and most problems

appeared to have been resolved. Next, some interviews

were subjected to behavior coding in order to identify

further difficulties with question wording. Since these

results demonstrated the persistence of a long-standing

problem, we sought an alternate strategy involving a

CATI programming change.

In addition, we discuss procedures used to contact

sample units and identify the respondent most

knowledgeable about the survey content. These

procedures include telephone screening to obtain the

respondent’s name, telephone number, fax number, and

current address, and sending advance letters to the

respondent. Results indicate that over 90% of the

interviews were conducted with the respondent

identified during screening. Screening also served to

assist in cleaning the data file. Screening also allowed

us to update and refine the sample files.

REFERENCES

Converse, J. and S. Presser. 1986. Survey Questions:

Handcrafting the Standardized Questionnaire.

Beverly Hills: Sage.

Cox, B.J. B.N. Chinnappa. 1995. "Unique Features of

Business Surveys." In B. Cox et al., eds., Business

Survey Methods. New York: Wiley, 1-17.

Edwards, W.S. D. Cantor. 1991. "Toward a Response

Model in Establishment Surveys." In P. Biemer et

al., Measurement Errors in Surveys. New York:

Wiley, 211-233.

Goldenberg, K. 1996. “Using Cognitive Testing in the

Design of a Business Survey Questionnaire.” Proceedings of the Section on Survey Research

Methods. American Statistical Association.

Griffiths, G. S. Linacre. 1995. “Quality Assurance for

Business Surveys.” In B.G. Cox et al., Business

Survey Methods. New York: Wiley, 673-690.

Groves, Robert M. 1989. Survey Errors and Survey

Costs. Wiley.

Levin, K., T. Shen, S. Heltemes, T. Hagerty, D.

Cantor, K. Goldenberg. 1996. Business Births:

Initial Evaluation of CATI Survey. Westat report

submit to the Bureau of Labor Statistics, August.

Moore, D. R. Baxter. 1993. “Increasing Mail Questionnaire Completion for Business Populations: The

effects of personalization and a telephone followup

procedure as elements of the Total Design Method,”

Proceedings of the International Conference on

Establishment Surveys. American Statistical Association, 496-502.

Oksenberg, L., C. Cannell, G. Kalton. 1991. “New

Strategies for Pretesting Questionnaires.” Journal of

Official Statistics 7: 349-365.

Paxson, M.C., D.A. Dillman, J. Tarnai. 1995. “Improving Response to Mail Surveys.” In B.G. Cox et al.,

Business Survey Methods. New York: Wiley, 303-

316.

Sudman, S., N.M. Bradburn, N. Schwarz. 1996. Thinking About Answers. San Francisco: Jossey-Bass.