Terminological aspects of the Guide to the Expression of Uncertainty in Measurement (GUM)

The Guide to the Expression of Uncertainty in Measurement (GUM) provided for the first time an international consensus on how to approach the widespread difficulties associated with conveying information about how reliable the value resulting from a measurement is thought to be. This paper examines the evolution in thinking and its impact on the terminology that accompanied the development of the GUM. Particular emphasis is put on the very clear distinction in the GUM between measurement uncertainty and measurement error, and on the reasons that even though 'true value' and 'error' are considered in the GUM to be 'unknowable' and, sometimes by implication, of little (or even no) use in measurement analysis, they remain as key concepts, especially when considering the objective of measurement. While probability theory in measurement analysis from a frequentist perspective was in widespread use prior to the publication of the GUM, a key underpinning principle of the GUM was to instead consider probability as a 'degree of belief.' The terminological changes necessary to make this transition are also covered. Even twenty years after the publication of the GUM, the scientific and metrology literatures sometimes contain uncertainty analyses, or discussions of measurement uncertainty, that are not terminologically consistent with the GUM, leading to the inability of readers to fully understand what has been done and what is intended in the associated measurements. This paper concludes with a discussion of the importance of using proper methodology and terminology for reporting measurement results.