Guide 98-3:2008, Guide to the Expression of Uncertainty in Measurement (GUM)

ISO and the International Electrotechnical Commission (IEC) have published the first of a planned series of guidance documents on the uncertainty of measurement.

ISO/IEC Guide 98-3:2008, Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995), is a reissue of the 1995 version of the Guide to the Expression of Uncertainty in Measurement (GUM), with minor corrections. The new guide is available in both paper and electronic form. It will be followed by the first of three supplements, ISO/IEC Guide 98-3/Supplement 1, Propagation of distributions using a Monte Carlo method. Further, ISO/IEC Guide 98-1 is being prepared to promote the sound evaluation of measurement uncertainty through the use of the GUM and to provide an introduction to the supplements and other related documents.

Measurement is involved in making commercial, health, environmental and scientific decisions. Measurement uncertainty enables users to obtain the probability of making an incorrect decision based on measurement, and to manage the consequential risks. Thus, a statement of measurement uncertainty is indispensable in judging the fitness for purpose of a value obtained by measurement.

As an example, a customer buying fruit at the greengrocer would be content if the scales gave a value within 2 g of the fruit's actual weight. However, the dimensions of parts of the gyroscopes within the inertial navigation systems of commercial aircraft must be checked by measurement to one part in a million for correct functioning.

ISO/IEC Guide 98-3 establishes 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. The principles of this guide are 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.

Though primarily concerned with the expression of uncertainty in the measurement of a well-defined physical quantity, the guide is also applicable to evaluating and expressing the uncertainty associated with the conceptual design and theoretical analysis of experiments, methods of measurement, and complex components and systems.

Since its initial publication in 1993, the GUM has found wide acceptance, and its methods have been adopted by various regional metrology and related organizations across the world. To quote from the introduction to ISO/IEC Guide 98-3: “Just as the nearly universal use of the International System of Units (SI) has brought coherence to all scientific and technological measurements, a worldwide consensus on the evaluation and expression of uncertainty in measurement permits the significance of a vast spectrum of measurement results in science, engineering, commerce, industry, and regulation to be readily understood and properly interpreted. In this era of the global marketplace, it is imperative that the method for evaluating and expressing uncertainty be uniform throughout the world so that measurements performed in different countries can be easily compared.”

The GUM is the responsibility of the Joint Committee for Guides in Metrology, which consists of experts nominated by the Bureau International des Poids et Mesures (BIPM), the International Electrotechnical Commission (IEC), the International Organization for Standardization (ISO), the International Organization of Legal Metrology (OIML), the International Federation for Clinical Chemistry and Laboratory Medicine (IFCC), the International Organization for Pure and Applied Chemistry (IUPAC), the International Organization for Pure and Applied Physics (IUPAP) and the International Laboratory Accreditation Cooperation (ILAC).