GUM (Guide to the Expression of Uncertainty in Measurement)

The Guide to the Expression of Uncertainty in Measurement, commonly known as the GUM, is the foundational document for uncertainty evaluation in metrology. Published by the Joint Committee for Guides in Metrology (JCGM), it provides a harmonized approach to quantifying measurement uncertainty that is accepted worldwide. The GUM replaced earlier methods that used terms like "error analysis" or "accuracy statements" with a more rigorous and consistent framework.

The GUM introduces the distinction between Type A evaluations (based on statistical analysis of observations) and Type B evaluations (based on other information such as specifications, calibration data, or scientific judgment). It describes how to model the measurement process, identify input quantities and their uncertainties, propagate uncertainties through the measurement model using partial derivatives (sensitivity coefficients), and express the result as an expanded uncertainty with a stated coverage factor.

For calibration professionals, the GUM is essential reading and the basis for all uncertainty analysis work. ISO 17025 requires that accredited laboratories follow GUM principles when evaluating measurement uncertainty. While the full mathematical treatment can be complex, the core concepts are accessible: identify what affects your measurement, quantify each effect, combine them properly, and state the result clearly. Supplement 1 to the GUM (JCGM 101) provides a Monte Carlo method as an alternative to the classical GUM approach for complex measurement models.