MEASURE FOR MEASURE
Write It Right
Understanding nuances of metrics in technical writing
by Elizabeth J. Gentry and Georgia L. Harris
Editor’s note: This is part two of a two-part series exploring accuracy in measurement results. Part one appeared in May 2016’s QP.
Many questions arise while you’re writing laboratory documents, clarifying measurement results or implementing measurement system best practices. The proper use of measurement units and symbols in laboratory documents—such as calibration reports, control charts, uncertainty tables or standard operating procedures—is critical to effectively communicate technical information.
The National Institute of Standards and Technology (NIST) was delegated the responsibility to interpret or modify the International System of Units (SI, also known as the metric system) for use in the United States. To accomplish this, NIST provides several SI resources to support sectors of science, technology, trade and commerce. It also serves as the U.S. 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.
NIST Special Publication (SP) 330 and NIST SP 811 provide the legal interpretation of and guidelines for SI use in the United States. NIST SP 811 also provides detailed rules for SI writing style, including a useful editorial checklist.1, 2
Striving for zero errors
NIST SP 811 is written for technical audiences, such as engineers, scientists and academics. Appendix B provides rounding guidance and unit-conversion factors for a broad set of measurement units. NIST published several similar technical guides, including the Metric Style Guide for the News Media, which provides condensed SI content to highlight commonly used measurement information.3 A convenient hub of SI style guidance also is available on the NIST metric program’s website.4
Use a leading zero: For numbers less than one, a zero is written before the decimal point.5 This ensures a quantity is appropriately interpreted and helps avoid consequences of a misplaced decimal point. Without a leading zero, a value like .25, for example, could be misinterpreted as 25, an error that makes it 100 times greater in magnitude. Such an error could seriously harm a patient if the quantity represented a medication dose.
Avoid unit-conversion errors: Using the SI reduces the number of errors associated with measurement conversions between U.S. customary units and the SI. Eliminating conversions altogether negates the need to document which conversion factors are being used and their sources. Conversion calculations require rigorous software validation, which is a time-consuming process. At best, conversion-calculation errors can cause expensive mistakes. At worst, their consequences can be a matter of life and death.
Ground control to accuracy
The 1999 crash of NASA’s $125 million NASA Mars climate orbital spacecraft served as a wake-up call for potential errors related to working with multiple measurement systems. The mishap occurred because the spacecraft entered the Mars atmosphere on a trajectory that was too low.6
NASA later identified the root cause of the erroneous trajectory and velocity calculations: A contractor failed to use SI units of force (Newton, or N) as specified by NASA in the coding of a ground software file used in trajectory models. One corrective action that NASA recommended was to perform software audits to evaluate specification compliance on all data transferred between NASA and the contractor.7
Several helpful conversion-factor resources have been made available on the NIST metric program’s website.8 Caution is recommended to organizations developing unit-conversion software or using online calculators for technical purposes. It’s important to conduct a rigorous validation and verification analysis before using unit-conversion software.
Spelling and pronunciation of measurement units—This can be challenging. Advantages of the SI over the many other historic and customary measurement unit systems is that the SI provides a coherent set of internationally accepted unit symbols that can be used to communicate across all languages. Table 1 provides examples of how unit names are translated in several languages:
In NIST SP 811, words are spelled in accordance with the U.S. Government Printing Office Style Manual (U.S. Government Publishing Office, 2008), which follows Webster’s Third New International Dictionary of the English Language (Merriam-Webster, 1993). 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.9 The text is published in French and English, and has been translated into many other languages.
Capitalization of units, symbols and prefixes—Unit names start with a lowercase letter except at the beginning of the sentence or title, such as "pascal," "becquerel," "newton" or "tesla." For degrees Celsius (symbol °C), the unit "degree" is lowercase. But the modifier "Celsius" is capitalized because it’s a person’s name. A space is left between the numerical value and the unit symbol, and values are not hyphenated. For example: 20 °C and 10 kg are correct; 20°C, 20° C, 10-kg or 10kg are incorrect. If a unit name is spelled out during use, normal grammar rules apply.
Unit symbols are written in lowercase letters (such as "m" for meter, "s" for second or "kg" for kilogram). But symbols for units derived from the name of a person are capitalized—such as W for watt, V for volt, Pa for pascal or K for kelvin. The recommended symbol for "liter" in the United States also is capitalized as L to avoid misinterpreting "l" with the number one. A period should not be used following a unit symbol or abbreviation. For example, gram is represented as "g" not "g." Symbols of prefixes that mean a million or more are capitalized, and those that are less than a million are lowercased. For example, M for mega (millions) and "m" for milli (thousandths).
U.S. customary units—After the SI was developed, many style requirements were applied to non-SI measurement systems, including U.S. customary units—such as inch, foot, yard, mile, ounce, pound, gill or gallon. Although NIST does not publish a style resource for U.S. customary units, appendix C of NIST’s Handbook 44, "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.10
Because the SI is critical as an international standard, its use in product design, manufacturing, marketing and labeling is essential for the U.S. industry’s success in the global marketplace. NIST’s metric program encourages using the SI in all facets of education, including honing workers’ skills.
The successful voluntary transition of the United States to the SI is a critical factor in the competitive economic success of industry.11 Accuracy in terminology use, measurement results and measurement units is necessary to avoid the embarrassment of having others find your "black dots" (errors that can negatively affect interpretations of your results in scientific communications). There are many resources that can help you avoid being responsible for inaccuracies in measurement reporting.
References and note
- Barry N. Taylor and Ambler Thompson, eds., The International System of Units (SI)—Special Publication 330, 2008 edition, National Institute of Standards and Technology (NIST), http://tinyurl.com/nistsp330.
- Barry N. Taylor and Ambler Thompson, eds., Guide for the Use of International System of Units (SI)—NIST Special Publication 811, 2008 edition, NIST, http://tinyurl.com/nist-sp811.
- NIST, Metric Style Guide for the News Media, 1997, http://tinyurl.com/metric-style-guide.
- "Writing With SI (Metric) Units," NIST.gov, http://tinyurl.com/si-writing.
- Taylor, Guide for the Use of International System of Units (SI)—NIST Special Publication 811, see reference 2.
- Robin Lloyd, "Metric Mishap Caused Loss of NASA Orbiter," CNN.com, Sept. 30, 1999, http://tinyurl.com/mars-metric.
- "Mars Climate Orbiter Mishap Investigation Board Phase I Report," NASA, Nov. 10, 1999, http://tinyurl.com/nasa-orbiter-report.
- "Unit Conversion," NIST.gov, http://tinyurl.com/nist-unit-conversion.
- International Bureau of Weights and Measures, The International System of Units (SI), eighth edition, 2006, http://tinyurl.com/bipm-si-brochure.
- Tina Butcher, Steve Cook, Linda Crown and Rick Harshman, eds., Specifications, Tolerances, and Other Technical Requirements for Weighting and Measuring Devices—NIST Handbook 44, NIST, 2012.
- If you have a question about metric system use, style or related publications, send it to email@example.com.
Elizabeth J. Gentry is metric coordinator for the National Institute of Standards and Technology (NIST), Office of Weights and Measures in Gaithersburg, MD. She earned a bachelor’s degree in biology from the University of Central Oklahoma in Edmond.
Georgia L. Harris is program leader at NIST, Office of Weights and Measures in Gaithersburg, MD. She earned a master’s degree in technical management from Johns Hopkins University in Baltimore. Harris is a senior member of ASQ and the committee secretary of the Measurement Quality Division.