The NIST Reference on Constants, Units and Uncertainty

Uncertainty of Measurement Results
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International and U.S. perspectives, continued

The Guide to the Expression of Uncertainty in Measurement
The end result of the work of ISO/TAG 4/WG 3 is the 100-page Guide to the Expression of Uncertainty in Measurement (or GUM as it is now often called). It was published in 1993 (corrected and reprinted in 1995) by ISO in the name of the seven international organizations that supported its development in ISO/TAG 4:

      BIPM Bureau International des Poids et Mesures
      IEC International Electrotechnical Commission
      IFCC International Federation of Clinical Chemistry
      ISO International Organization for Standardization
      IUPAC International Union of Pure and Applied Chemistry
      IUPAP International Union of Pure and Applied Physics
      OIML International Organization of Legal Metrology

       The focus of the ISO Guide or GUM is the establishment of "general rules for evaluating and expressing uncertainty in measurement that can be followed at various levels of accuracy and in many fields--from the shop floor to fundamental research." As a consequence, the principles of the GUM are intended to be applicable to a broad spectrum of measurements, including those required for:

    maintaining quality control and quality assurance in production;

    complying with and enforcing laws and regulations;

    conducting basic research, and applied research and development, in science and engineering;

    calibrating standards and instruments and performing tests throughout a national measurement system in order to achieve traceability to national standards;

    developing, maintaining, and comparing international and national physical reference standards, including reference materials.

Wide acceptance of the GUM
The GUM has found wide acceptance in the United States and other countries. For example:

    The GUM method of evaluating and expressing measurement uncertainty has been adopted widely by U.S. industry as well as companies abroad.

    The National Conference of Standards Laboratories (NCSL), which has some 1500 members, has prepared and widely distributed Recommended Practice RP-12, Determining and Reporting Measurement Uncertainties, based on the GUM.

    ISO published the French translation of the GUM in 1995, German and Chinese translations were also published in 1995, and an Italian translation was published in 1997. Translations of the GUM into Estonian, Hungarian, Italian, Japanese, Spanish, and Russian have been completed or are well underway.

    GUM methods have been adopted by various regional metrology and related organizations including:

      NORAMET  North American Collaboration in Measurement Standards
      NAVLAP National Voluntary Laboratory Accreditation Program
      A2LA American Association for Laboratory Accreditation
      EUROMET European Collaboration in Measurement Standards
      EUROLAB A focus for analytic chemistry in Europe
      EA European Cooperation for Accreditation
      EU European Union; adopted by CEN and published as EN 13005.

       Moreover, the GUM has been adopted by NIST and most of NIST's sister national metrology institutes throughout the world, such as the National Research Council (NRC) in Canada, the National Physical Laboratory (NPL) in the United Kingdom, and the Physikalisch-Technische Bundesanstalt in Germany.

       Most recently, the GUM has been adopted by the American National Standards Institute (ANSI) as an American National Standard. Its official designation is ANSI/NCSL Z540-2-1997 and its full title is American National Standard for Expressing Uncertainty--U.S. Guide to the Expression of Uncertainty in Measurement. This publication may be ordered directly from NCSL.

       It is noteworthy that NIST's adoption of the GUM approach to expressing measurement uncertainty was done with considerable forethought. Although quantitative statements of uncertainty had accompanied most NIST measurement results, there was never a uniform approach at NIST to the expression of uncertainty. Recognizing that the use of a single approach within NIST instead of a variety of approaches would simplify the interpretation of NIST outputs, and that U.S. industry was calling for a uniform method of expressing measurement uncertainty, in 1992 then NIST Director J. W. Lyons appointed a NIST Ad Hoc Committee on Uncertainty Statements to study the issue. In particular, the Ad Hoc committee was asked to ascertain if the GUM approach would meet the needs of NIST's customers. The conclusion was that it most definitely would, and a specific policy for the implementation of the GUM approach at NIST was subsequently adopted.

       NIST Technical Note 1297 (TN 1297, online in a pdf version or in an html version -- see the Bibliography for full citation) was prepared by two members of the Ad Hoc Committee, who also played major roles in the preparation of the GUM. (The policy, "Statement of Uncertainty Associated with Measurement Results," was incorporated in the NIST Administrative Manual and is included as Appendix C in TN 1297.) TN 1297 has in fact found broad acceptance. To date, over 40 000 copies have been distributed to NIST staff and in the United States at large and abroad -- to metrologists, scientists, engineers, statisticians, and others who are involved with measurement in some way.

       Most recently, a new international organization has been formed to assume responsibility for the maintenance and revision of the GUM and its companion document the VIM (see the Bibliography for a brief discussion of the VIM). The name of the organization is Joint Committee for Guides in Metrology (JCGM) and its members are the seven international organizations listed above: BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, and OIML, together with the International Laboratory Accreditation Cooperation (ILAC). ISO/TAG 4 has been reconstituted as the Joint ISO/IEC TAG, Metrology, and will focus on metrological issues internal to ISO and IEC as well as represent ISO and IEC on the JCGM.


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