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Weights and Measures Connection, Vol. 7, Issue 4 Page 1

7/1/2016

Measure for Measure – Does Accuracy Matter to You?

Byline: Elizabeth J. Gentry, NIST Office of Weights and Measures U.S. Metric Coordinator, and

Georgia L. Harris, NIST Office of Weights and Measures, Laboratory Metrology Program Leader

Accuracy in Measurement Results 

Many decisions are based on accurate measurement results. Should medicine be prescribed for high 

cholesterol or high glucose? Should a measuring instrument or standard be adjusted to meet tolerances? 

Is there really global warming or climate change? Answers to these questions are all based on 

measurement results and as a patient, scientist, citizen, or policy maker, we make assumptions about the 

accuracy of measurement results we receive in reports and calibration certificates. We assume they are 

good, that is right, or more correctly accurate. Note that accuracy is often defined as hitting the center of 

the target or a true value. A colleague of the authors was noted to regularly say “the only true value is on 

a sign above a hardware store.” However, measurement users often trust the accuracy of measurement 

results they receive, usually without question. Users of measurement results believe that the results are 

of good quality and are good and right.

However, a measurement result alone is incomplete without some assessment and measure of reported 

uncertainty associated with the result. People can estimate the temperature outside on a warm spring 

day within a few degrees based on their experience and scientists can estimate the temperature in a 

laboratory within a degree based on their experience working in environmentally controlled spaces. 

However, using a thermometer, we first hope that it is accurate and giving us the correct or right 

temperature, but then we must consider the resolution of the standard. Is the readability of the 

thermometer 1 °C, 0.1 °C, or 0.01 °C. Our confidence in the results, if they are right, will be dependent 

upon the readability or resolution of the standard or measuring instrument. Our confidence should not 

be based on the fact that a calculator or spreadsheet will give us a calculated value to 15 decimal places 

when the resolution or uncertainty is only to 1 or 2 decimal places!

Repeatability of the instrument or standard is also a variable of concern. How often do people naturally 

repeat measurements to get a sense of whether multiple values agree? Even simple measurements such 

as stepping on a scale to get an idea of one’s weight (where we always hope the second or third reading 

will be lower), inserting a kitchen thermometer into the turkey to see if it has reach the desired 

temperature, or checking our mileage over time to calculate fuel efficiency for our vehicles, are all 

common in daily life. 

In a calibration laboratory, the assignment of uncertainty to the measurement result is a rigorous, 

documented, and validated process that is assessed nearly as often as the measurement results 

themselves. Measurement scientists often use internationally accepted procedures to obtain 
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standardized measurement results, and use the Guide to the Expression of Uncertainty in Measurement1

as a guiding document on evaluating and reporting measurement results and their associated

uncertainties. 

The readability or resolution and repeatability of the measurement results give us a sense of confidence, 

or lack of confidence, and is a measure of the uncertainty associated with the measurement results. It is 

a wise practice to ask for the measurement uncertainty and use it as an assessment of the quality of a 

measurement result and its precision. Uncertainty values provide a measure of confidence in the 

measurement result; it provides a quantification of the boundaries or limits within which the 

measurement result should agree with a true quantity value.

Accuracy in Terminology and Communications

Once accurate measurement results and associated uncertainties are available, they need to be 

communicated. Communications may be in a newspaper, on television, in a scientific paper, or on a 

calibration certificate. Thus, accuracy in our words and measurement results is also critical. As in the case 

of measurement uncertainties, measurement scientists also have guiding documents to help standardize 

communications. In this case, the International Vocabulary of Metrology (VIM)2 is used as the guiding 

document on terms used with measurement and calibration results. Uses of terms like accuracy, 

traceability, uncertainty, and reference standards all have very specific meaning to measurement 

professionals and should be used by all scientists. 

For example, the VIM defines accuracy, as related to a measurement result, as:

2.13 (3.5) 

measurement accuracy 

accuracy of measurement 

accuracy 

closeness of agreement between a measured quantity value and a true quantity value of a measurand 

NOTE 1 The concept “measurement accuracy” is not a quantity and is not given a numerical quantity value. 

A measurement is said to be more accurate when it offers a smaller measurement error. 

NOTE 2 The term “measurement accuracy” should not be used for measurement trueness and the term 

“measurement precision” should not be used for “measurement accuracy,” which, however, is related to 

both these concepts. 

 1 Evaluation of Measurement Data — Guide to the Expression of Uncertainty in Measurement, JCGM 100:2008 GUM 

1995 with minor corrections, available free at: http://www.bipm.org/en/publications/guides/gum.html, 2015-01-

12. 

2 International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM 3rd edition)

JCGM 200:2012 (JCGM 200:2008 with minor corrections), available for free: 

http://www.bipm.org/en/publications/guides/vim.html, 2015-01-12.
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NOTE 3 “Measurement accuracy” is sometimes understood as closeness of agreement between measured 

quantity values that are being attributed to the measurand.

According to the VIM, measurand is “the quantity intended to be measured.” But, it becomes obvious

that when comparing measurement results from one laboratory or between countries, scientists must be 

able to talk about the same thing. Thus, the use of standardized definitions is essential. As the reader 

can see, even the definition of accuracy requires explanatory notes! Use of standardized terminology can 

help avoid a Tower of Babel Effect3 when communicating measurement results!

Accuracy in Presenting Units, Symbols, and Measurement Results

Finally, measurement results must be communicated with proper quantities, units, and symbols. In fact, 

many countries adopt the International System of Units (SI) as the reference basis for measurement 

results. Again, there is a standardized document for referencing and presenting measurement units, 

symbols, and results.4

 The U.S. Metric Program of the National Institute of Standards and Technology 

(NIST), Office of Weights and Measures (OWM), helps implement the national policy to establish the SI 

(International System of Units, commonly known as the metric system) as the preferred system of weights 

and measures for U.S. trade and commerce. It provides leadership and assistance on SI use and 

conversion to federal agencies, state and local governments, businesses, trade association, standards 

development organizations, educators, and the general public. Together NIST Special Publication (SP) 

330, The International System of Units (SI), and NIST SP 811,5 Guide for the Use of the International System 

of Units (SI), provide the legal interpretation of and guidelines for the use of the SI in the United States, 

as described in the Interpretation of the International System of Units (SI) for the United States. These 

publications provide standardized guidance on how measurement units and results should be presented 

in writing.

Black Dots

One of the authors is often quoted as saying “if we [measurement scientists] don’t get communication of 

measurement results right, who will?” Regular review of measurement results and measurement 

uncertainties on calibration certificates and in laboratory documents yield numerous errors that can 

negatively impact the interpretation of the results by a user. Examples are often observed where 

measurement uncertainties are not included, are incomplete, or are inaccurate, incorrect terminology is 

used, typographical errors are left uncorrected, results are presented without appropriate significant 

digits, unit conversions are made inaccurately, and incorrect units and symbols are presented or correct 

units are inconsistently used. The authors refer to these errors in communications as black dots. That is, 

a black dot on a clear page is what is noticed and is obvious by the reader or customer. These types of 

 3 In the Bible, a city (now thought to be Babylon) in Shinar where God confounded a presumptuous attempt to build 

a tower into heaven by confusing the language of its builders into many mutually incomprehensible languages.

(http://www.thefreedictionary.com/Babel+effect, 2015-01-12.) 4 SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014], available for free: 

http://www.bipm.org/en/publications/si-brochure/, 2015-01-12. 5 The International System of Units (SI), NIST SP 330, Edition: 2008. Guide for the Use of International System of Units 

(SI), NIST SP 811, Edition: 2008. Available for free: http://www.nist.gov/pml/wmd/metric/metric-pubs.cfm, 2015-

01-12.
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blemishes will be what are remembered, despite the presence of other accurate information that’s 

presented. Errors in reporting results can lead to confusion or bad decisions by the user, often with critical 

impacts. Black dots destroy credibility of the provider of the measurement results. 

Examples of common black dots, as ripped from the headlines or as readily observed in daily life, include6

: 

• Failure to include leading zeroes when presenting measurement results so that a child is given a 

5 mL dosage instead of a 0.5 mL dosage leading to death;

• Failure to effectively communicate requirements and convert measurement units when using two 

measurement systems lead to the loss of the NASA Mars Climate Orbiter; 

• The 2003 Disneyland Tokyo Space Mountain roller coaster accident highlights a scenario where 

axle and bearing design specifications were converted to metric units and implemented in an 

amusement ride. After the design change took place, time passed and routine maintenance called 

for the replaced bearings. However, instead of being replaced with the updated design (metric) 

bearings, they were replaced with the incorrect size based on the original (non-metric) design. 

Use of the wrong size bearing created a gap between the axle and bearing. Over time, extra 

vibration and stress cause the axle to fail and the roller coaster to derail. Luckily, no passengers 

were injured during the ride!7

• Recently, a measurement calculation mistake was discovered in a laminate wood flooring study 

conducted by the U.S. Centers for Disease Control and Prevention (CDC). The federal agency 

issued a correction notice to a report about the quantity of formaldehyde in the product revealing 

that, as result of not converting from feet to meters, an incorrect value for ceiling height caused 

CDC scientists to significantly underestimate the health risks in the original study report.8,9

 

• The NIST Office of Weights and Measures (OWM) conducted a labeling assessment of over 1100 

packages, including products food, home and personal care products, hobby and arts and crafts 

products, automotive, hardware, office products, and pet supplies and observed that some 

inappropriate unit symbols were in use within the net quantity of contents statements.10 OWM 

 6 U.S. Metric Association, Unit Mixups (2016-02-26) http://www.us-metric.org/unit-mixups/

7 U.C. Davis, ChemWiki, Case Studies: Metric/English Conversion Errors (2016-02-26) 

http://chemwiki.ucdavis.edu/Core/Analytical_Chemistry/Quantifying_Nature/Case_Studies%3A_Metric%2F%2FEn

glish_Conversion_Errors#Disneyland_Tokyo:_A_Bumpy_Blunder

8 CDC Fixes Major Error in Flooring Risk Report: Not Converting to Metric, Retraction Watch (2016-02-24) 

http://retractionwatch.com/2016/02/22/cdc-fixes-major-error-in-flooring-risk-report-not-converting-to-metric/

9 CDC Revises Health Risk Assessment of Flooring After Math Error, Vocativ (2016-02-24) 

http://www.vocativ.com/news/289002/cdc-laminate-cancer/

10 NIST OWM Marketplace Assessment - Metric Labeling on Packages in Retail Stores (December 2009) 

http://www.nist.gov/pml/wmd/metric/upload/Marketplace-Assessment-Metric-Labeling-Retail-StoresDec2009.pdf
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offers packaging and labeling training opportunities to help packers and retailers become more 

familiar with state and federal labeling requirements. 

Document control is also important within both laboratory and a wide variety of operational 

environments. Version control and archiving records are essential tools to ensure changes made over 

time are effectively communicated to all personnel impacted by a change. Failure to maintain adequate 

control of laboratory documents, for example calibration certificate templates, can be the root cause for 

black dots that are released to customers. 

One goal of ensuring quality when communicating accurate measurement results should always be to 

avoid black dots! Quality communications of measurement results require a rigorous review of all 

communications for use of accurate terminology, completeness, and use of appropriate measurement 

units and symbols in addition to editorial reviews for typographical and grammatical errors. The use of 

the publications noted in this article will guide the user to standardized resources for improving the 

quality, accuracy, and communication of measurement results. 

Avoid the Black Dot: Know Where to Get the Answers

Knowing where to find the answers to common technical questions can be invaluable. Many questions 

arise when writing laboratory documents, clarifying measurement results, and implementing 

measurement system best practices. The proper use of measurement units and symbols in laboratory 

documents, such as calibration reports, control charts, uncertainty tables, and standard operating 

procedures (SOP) is critical to effectively communicating technical information both internally between 

laboratory personnel and externally with customers.

Congress has assigned the responsibility to 

interpret or modify the SI (International System 

of Units) for use in the United States to the 

Secretary of Commerce. This responsibility has 

been delegated by the Secretary to NIST. To 

accomplish this mission, NIST provides a number 

of SI resources and information to support 

science, technology, trade, and commerce. 

NIST Special Publication (SP) 811, Guide for the 

Use of the International System of Units (SI), 

provides11 detailed rules for SI writing style, 

including a useful Editorial Checklist at the 

beginning of the document. 

Together NIST Special Publication (SP) 330, International System of Units (SI), and SP 811 provide the legal 

interpretation of and guidelines for the use of the SI in the United States. NIST also serves as the U.S. 

 11 NIST SP 811, Guide for the Use of the International System of Units (SI), (2016-02-24), 

http://www.nist.gov/pml/pubs/sp811/indexfull.cfm

 

Figure 1. NIST SP 330 and SP 811 – Key technical SI resources.
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technical representative to the International Bureau of Weights and Measures (BIPM) that defines the SI. 

These publications are used to guide the measurement unit style in technical and documentary standards.

SP 811 is written for technical professional audiences, including engineers, scientists, and academics. 

Appendix B provides rounding guidance as well as Unit Conversion Factors for a broad set of measurement 

units. NIST has produced several other similar technical guides for a diverse group of audiences, including 

LC 1137, Metric Style Guide for the News Media, which provides condensed SI content to highlight 

commonly used measurement information.12 A convenient hub of SI style guidance is available on the 

NIST Metric Program’s “Writing with Metric Units” website.13

Use a Leading Zero. It’s appropriate to use what’s known as a leading zero before the decimal marker to 

ensure the quantity is appropriately interpreted and avoid the consequences of a misplaced decimal 

point. For numbers less than one, a zero is written before the decimal marker. 14 Without a leading zero, 

a value like .25 might be visually misinterpreted as 25, an error of 100 times greater magnitude. For 

example, if the quantity represented a pharmaceutical medication dose, it’s highly likely that harm or a 

worse health effect would result from the error.

Avoid Unit Conversions Errors. A fundamental benefit of SI 

use is a reduction of errors due to measurement conversions 

between U.S. customary units and the SI. Eliminating 

conversions altogether can negate the need to document 

which conversion factors are being used and their source. 

Conversion calculations require rigorous software validation, 

which is an expensive and time consuming process. At best, 

the price of conversion calculation errors can be expensive. At 

worst, consequences can be a matter of life and death.

The crash of the $125 million NASA Mars Climate Orbital 

spacecraft in 1999 was a wake-up call for potential errors 

related to working with multiple measurement systems. The 

mishap occurred when the spacecraft entered the Mars atmosphere on a trajectory that was too low.15 

NASA later determined that the root cause of the erroneous trajectory and velocity calculations were due 

to the contractor failing to use SI units of force (Newton, N), as specified by NASA, within the coding of a 

ground software file used in trajectory models. One corrective action recommended by NASA was a 

 12 NIST LC 1137, Metric Style Guide for the News Media (2016-02-26)

13 NIST Metric Program, Writing with Metric Units (2016-02-24) http://www.nist.gov/pml/wmd/metric/writingmetric.cfm

14 NIST SP 811, Guide for the Use of the International System of Units (SI), Section 10.5.2 (2016-02-24), 

http://www.nist.gov/pml/pubs/sp811/indexfull.cfm

15 CNN, Metric Mishap Caused Loss of NASA Orbiter (2016-02-26) 

http://www.cnn.com/TECH/space/9909/30/mars.metric.02/

Figure 2. NASA Mars Obiter.
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software audit to evaluate specification compliance on all data transferred between NASA and the 

contractor.16

Several helpful conversion factor resources have been made available on the NIST Metric Program’s “Unit 

Conversion” website.17 Caution is recommended to those developing unit conversion software or using 

an online calculator for technical purposes. It’s important to conduct a rigorous validation and verification 

analysis before implementing unit conversion software.

Spelling and Pronunciation. Spelling and pronunciation of measurement units can also be challenging 

topics in practice. One of the advantages of the SI over the many other historic and customary 

measurement unit systems is that although unit names and prefixes may be spelled differently in different 

languages, the SI provides a coherent set of internationally accepted unit symbols that can be used to 

communicate across all languages. For example, the names of the units themselves vary in spelling and 

pronunciation according to national practices.18

Unit Symbol

Language m s kg

English (U.S) meter second kilogram

Spanish metro segundo kilogramo

Italian metro secondo chilogrammo

In NIST SP 811, the spelling of English words is made in accordance with the U.S. Government Printing 

Office Style Manual, which follows Webster’s Third New International Dictionary rather than the Oxford 

Dictionary. The spellings “meter,” “liter,” and “deka” are used rather than “metre,” “litre,” and “deca” as 

in the original BIPM English text of the SI Brochure.

The BIPM SI Brochure is the definitive international reference on the SI and is published by the 

International Bureau of Weights and Measures (BIPM).19 The text is published in French language (official 

version), followed by the English language text. Neither the French or English text provide pronunciation 

guidance. The SI Brochure content is widely translated into many other languages, which may also explain 

why pronunciation isn’t specifically addressed. Interestingly, the International Vocabulary of Metrology

(VIM) doesn’t address pronunciation.20

 16 National Aeronautics and Space Administration (NASA), Mars Climate Orbiter Mishap Investigation Board Phase I 

Report, November 10, 1999.

17 NIST Metric Program, Unit Conversion (2016-02-24) http://www.nist.gov/pml/wmd/metric/unit-conversion.cfm

18 Metric Methods, SI Crosses All Language Barriers (2016-02-26) 

http://metricmethods.com/Multilingual_SI_unit_names.gif

19 Bureau International des Poids et Mesures/ International Bureau of Weights and Measures, SI Brochure 

(2016-02-26), http://www.bipm.org/en/publications/si-brochure/

20 VIM3: International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (2016-02-26) 

http://www.bipm.org/en/publications/guides/vim.html
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Capitalization of Units, Symbols, and Prefixes. The names of all units start with a lower case letter except 

at the beginning of the sentence or title, for example: pascal, becquerel, newton, tesla. In the example 

for degree Celsius (symbol °C), the unit “degree” is lower case but the modifier “Celsius” is capitalized 

because it is the name of a person. A space is left between the numerical value and unit symbol and 

values are not hyphenated (20 °C and 10 kg are correct; 20°C, 20° C, 10-kg or 10kg are not correct). If the 

unit name is spelled out during use, then normal grammar rules are applied.

Unit symbols are written in lower case letters (e.g., m for meter, s for second, kg for kilogram). However, 

symbols for units derived from the name of a person are capitalized (W for watt, V for volt, Pa for pascal, 

K for kelvin, etc.). The recommended symbol for the liter in the United States is also capitalized as L to 

avoid misinterpreting l (lowercase L) with the numeral 1. The period punctuation mark should not be 

used following a unit symbol (or abbreviation). For example, gram is represented “g” not “g.” (g period). 

Symbols of prefixes that mean a million or more are capitalized and those less than a million are lower 

case. For example, M for mega (millions), m for milli (thousandths). 

U.S. Customary Units. Since the development of the SI, many of the style requirements have been applied 

to non-SI measurement systems, including U.S. Customary units (e.g., inch, feet, yard, mile, ounce, pound, 

fluid ounce, gill, gallon). Although NIST does not publish a style resource for U.S. Customary units, NIST 

Handbook (HB) 44, Specifications, Tolerance, and Other Technical Requirements for Weighing and 

Measuring Devices, Appendix C. General Tables of Units of Measurement is a good resource for U.S. 

Customary units used in trade and commerce, their relationships, and unit conversion factors.

Because of the importance of the SI (metric system) as an international standard, its use in product design, 

manufacturing, marketing, and labeling is essential for U.S. industry’s success in the global marketplace. 

The NIST Metric Program encourages the use of the SI in all facets of education, including honing of worker 

skills. The successful voluntary transition of the United States to the SI is a critical factor in the competitive 

economic success of industry. Questions regarding metric system (SI) use, style, or related publications 

can be directed to TheSI@nist.gov. 

Accuracy in the use of terminology, measurement results, and measurement units is critical for avoiding 

the embarrassment of having others find black dots in your scientific communications. Numerous 

resources are available to help you avoid being responsible for inaccuracies reported on calibration 

certificate and the next problematic news headline regarding the inaccurate use of measurement units.