Guide to the Expression of Uncertainty in Measurement

Guide to the Expression of Uncertainty in Measurement. It is a foundational metrological document providing a unified, internationally accepted framework for quantifying and reporting measurement uncertainty. Developed through the collaboration of major international standards bodies, it aims to ensure consistency and comparability of measurement results across scientific and industrial disciplines. Its principles are now embedded in global standards for laboratory accreditation and quality assurance.

Overview and purpose

The development of the GUM was initiated by the International Bureau of Weights and Measures in response to the need for a harmonized approach to uncertainty, a need highlighted by international comparisons like those coordinated by the Consultative Committee for Units. Prior to its publication, various organizations, including the National Institute of Standards and Technology in the United States and national metrology institutes in countries like the United Kingdom and Germany, used differing conventions. The primary purpose is to establish a general methodology applicable to a wide range of measurements, from fundamental research at institutions like CERN to industrial testing and clinical diagnostics. This harmonization supports international trade, regulatory compliance under bodies like the Food and Drug Administration, and scientific collaboration.

Basic concepts and definitions

The GUM introduces a standardized vocabulary, central to which is the distinction between Type A evaluation of uncertainty and Type B evaluation of uncertainty. A key concept is the standard uncertainty, which is expressed as the standard deviation of the probability distribution associated with the measurement result. The guide defines the combined standard uncertainty, obtained by combining individual uncertainty components using the law of propagation of uncertainty, often based on the Taylor series expansion. For providing an interval with a high level of confidence, it defines the expanded uncertainty, calculated using a coverage factor typically linked to the Student's t-distribution. The underlying philosophy treats all uncertainty components, regardless of origin, by their probability distributions.

Methods for evaluating uncertainty

For Type A evaluation, methods are based on statistical analysis of series of observations, employing tools like calculating the experimental standard deviation of the mean. For Type B evaluation, scientific judgment is used, drawing on information such as calibration certificates from a National Metrology Institute, data from handbooks like the CRC Handbook of Chemistry and Physics, or manufacturer specifications for equipment from companies like Keysight Technologies or Mettler Toledo. The GUM details the process of constructing a measurement model, identifying all input quantities, and applying the law of propagation. A major supplement, JCGM 101:2008, later introduced a more robust numerical approach using Monte Carlo method simulations for complex models.

Reporting uncertainty

The GUM provides explicit rules for reporting a complete measurement result. The result must be stated as a single estimated value accompanied by its combined standard uncertainty or expanded uncertainty, with the coverage factor or confidence level clearly specified. This format is now a mandatory requirement in standards such as ISO/IEC 17025 for testing and calibration laboratories accredited by bodies like the American Association for Laboratory Accreditation. Consistent reporting allows for proper comparison with specification limits, assessment against regulations from the Environmental Protection Agency, and meaningful interpretation in contexts like interlaboratory studies organized by the AOAC International.

Applications and impact

The GUM's impact is profound and global, forming the bedrock of modern metrology. Its application is required in virtually every accredited calibration laboratory worldwide, influencing work at institutions ranging from the National Physical Laboratory in the UK to the Physikalisch-Technische Bundesanstalt in Germany. It is essential in fields requiring high measurement integrity, such as pharmaceutical manufacturing monitored by the European Medicines Agency, aerospace engineering involving NASA, and forensic science. The guide has been adopted by major standards organizations, including the International Organization for Standardization and the International Electrotechnical Commission, and its principles are taught in universities and codified in influential documents like the EURAMET guides.

Category:Metrology Category:International standards Category:Measurement