Measurement uncertainty and its connection with true value in the GUM versus JCGM documents

This paper discusses the concept of uncertainty in measurement and its connection with true value as defined in the Guide to the Expression of Uncertainty in Measurement (GUM), and three subsequent documents from the Joint Committee for Guides in Metrology (JCGM): JCGM 101 (GUM-S1), JCGM 104, and JCGM 200 (VIM3). An ordinary measurand is specified by its description. The GUM and the JCGM 200 explain that every ordinary measurand has a range of true values. The GUM requires that the measurand should be described in sufficient detail with the consequence that the range of its true values is small enough for it to be adequately characterized by a single scalar measured value with standard uncertainty. The concept of uncertainty in measurement as established by the GUM does not refer to the theoretical concepts of true value and error. So, it applies whether the measurand has a unique true value or a range of true values. The JCGM 101 has introduced a concept of uncertainty in measurement called a coverage interval with a stated coverage probability. The concept of a coverage interval requires the measurand to have a unique true value, so it does not apply to an ordinary measurand that has a range of true values. Most measurands of interest in metrology are ordinary measurands; therefore, the JCGM idea of a coverage interval is of little use.