Measurement uncertainty redefined

Abstract On July 2, 2025, the Joint Committee for Guides in Metrology (JCGM) organized a widely attended webinar to present and discuss a new definition of measurement uncertainty to be considered as a potential replacement for the current definition in the International Vocabulary of Metrology. The original version of the new definition first appeared in Possolo (2015 Simple Guide for Evaluating and Expressing the Uncertainty of NIST Measurement Results (National Institute of Standards and Technology)) and was adopted by the National Institute of Standards and Technology (NIST) of the United States: ‘Measurement uncertainty is the doubt about the true value of the measurand that remains after making a measurement. Measurement uncertainty is described fully and quantitatively by a probability distribution on the set of the measurand.’ This definition was inspired by an earlier, similar characterization of measurement uncertainty that Stephanie Bell (National Physical Laboratory, United Kingdom) published in 1999 (Bell 1999 A Beginner’s Guide to Uncertainty of Measurement (Measurement Good Practice Guide) vol 11 (National Physical Laboratory)). The present short communication first compares definitions of measurement uncertainty and reviews the ‘error’ and ‘uncertainty’ approaches featured in the introduction to the third edition of the International Vocabulary of Metrology. Next, it explains the principal motivation for the redefinition, which was a critical review of the current definition by Thompson (2011 Accr. Qual. Assur. 17 93–94). The third section discusses the relation between doubt, incomplete knowledge, and uncertainty, and the concern that words like ‘doubt’ and ‘uncertainty’ can cast a negative light upon the science and technology that produce measurement results, in particular in laboratory medicine. A closing Coda highlights the principal benefits of the proposed redefinition.