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Vol. 30 No. 6
November-December 2008

Bookworm | Books and publications hot off the press.
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International Vocabulary of Metrology—Basic and General Concepts and Associated Terms

3rd edition, JCGM (Joint Committee for Guides in Metrology) 200:2008

In general, a vocabulary is a “terminological dictionary which contains designations and definitions from one or more specific subject fields” (ISO 1087-1:2000, 3.7.2). The vocabulary in this book pertains to metrology, the “science of measurement and its application.” It also covers the basic principles governing quantities and units. The field of quantities and units could be treated in many different ways. Clause 1 of this vocabulary is one such treatment, and is based on the principles laid down in the various parts of ISO 31, Quantities and Units, currently being replaced by the ISO 80000 and IEC 80000 series Quantities and Units, and in the SI brochure, The International System of Units (published by the Bureau International des Poids et Mesures, BIPM).

The second edition of the International Vocabulary of Basic and General Terms in Metrology (VIM) was published in 1993. A third edition was published in order to cover measurements in chemistry and laboratory medicine for the first time, as well as to incorporate concepts related to metrological traceability, measurement uncertainty, and nominal properties. Its title is now International Vocabulary of Metrology—Basic and General Concepts and Associated Terms (VIM) in order to emphasize the primary role of concepts in developing a vocabulary.
In this vocabulary, it is taken for granted that there is no fundamental difference among the basic principles of measurement in physics, chemistry, laboratory medicine, biology, or engineering. Furthermore, an attempt has been made to meet conceptual needs of measurement in fields such as biochemistry, food science, forensic science, and molecular biology.

Part of a diagram from the International Vocabulary of Basic and General Terms in Metrology.

Several concepts that appeared in the second edition of the VIM do not appear in this third edition because they are no longer considered to be basic or general. For example, the concept “response time,” used in describing the temporal behavior of a measuring system, is not included. For concepts related to measurement devices that are not covered by this third edition of the VIM, the reader should consult other vocabularies such as IEC 60050, International Electrotechnical Vocabulary. For concepts concerned with quality management, mutual recognition arrangements pertaining to metrology, or legal metrology, the reader is referred to documents given in the bibliography.

Development of this third edition of the VIM has raised some fundamental questions about different current philosophies and descriptions of measurement, as will be summarized below. These differences sometimes lead to difficulties in developing definitions that could be used across the different descriptions. No preference is given in this third edition to any of the particular approaches.

The change in the treatment of measurement uncertainty from an Error Approach (sometimes called Traditional Approach or True Value Approach) to an Uncertainty Approach necessitated reconsideration of some of the related concepts appearing in the second edition of the VIM. The objective of measurement in the Error Approach is to determine an estimate of the true value that is as close as possible to that single true value. The deviation from the true value is composed of random and systematic errors. The two kinds of errors, assumed to be always distinguishable, have to be treated differently. No rule can be derived on how they combine to form the total error of any given measurement result, usually taken as the estimate. Usually, only an upper limit of the absolute value of the total error is estimated, sometimes loosely named “uncertainty.”

In the CIPM Recommendation INC-1 (1980) on the Statement of Uncertainties, it is suggested that the components of measurement uncertainty should be grouped into two categories, Type A and Type B, according to whether they were evaluated by statistical methods or otherwise, and that they be combined to yield a variance according to the rules of mathematical probability theory by also treating the Type B components in terms of variances. The resulting standard deviation is an expression of a measurement uncertainty. A view of the Uncertainty Approach was detailed in the Guide to the Expression of Uncertainty in Measurement (GUM) (1993, corrected and reprinted in 1995) that focused on the mathematical treatment of measurement uncertainty through an explicit measurement model under the assumption that the measurand can be characterized by an essentially unique value. Moreover, in the GUM as well as in IEC documents, guidance is provided on the Uncertainty Approach in the case of a single reading of a calibrated instrument, a situation normally met in industrial metrology.

The objective of measurement in the Uncertainty Approach is not to determine a true value as closely as possible. Rather, it is assumed that the information from measurement only permits assignment of an interval of reasonable values to the measurand, based on the assumption that no mistakes have been made in performing the measurement. Additional relevant information may reduce the range of the interval of values that can reasonably be attributed to the measurand. However, even the most refined measurement cannot reduce the interval to a single value because of the finite amount of detail in the definition of a measurand. The definitional uncertainty, therefore, sets a minimum limit to any measurement uncertainty. The interval can be represented by one of its values, called a “measured quantity value.”

In the GUM, the definitional uncertainty is considered to be negligible with respect to the other components of measurement uncertainty. The objective of measurement is then to establish a probability that this essentially unique value lies within an interval of measured quantity values, based on the information available from measurement.

The IEC scenario focuses on measurements with single readings, permitting the investigation of whether quantities vary in time by demonstrating whether measurement results are compatible. The IEC view also allows non-negligible definitional uncertainties. The validity of the measurement results is highly dependent on the metrological properties of the instrument as demonstrated by its calibration. The interval of values offered to describe the measurand is the interval of values of measurement standards that would have given the same indications.

In the GUM, the concept of true value is kept for describing the objective of measurement, but the adjective “true” is considered to be redundant. The IEC does not use the concept to describe this objective. In this vocabulary, the concept and term are retained because of common usage and the importance of the concept.

See earlier announcement about the availability of the VIM in the Sep-Oct 2008 CI, page 18. This 3rd edition is also published on paper by ISO (ISO/IEC Guide 99-12:2007, International Vocabulary of Metrology-Basic and General Concepts and Associated Terms, details are available at <www.iso.org>).

The full text of the VIM is available for free online at
<www.bipm.org/en/publications/guides/vim.html>.


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