True value and uncertainty in the GUM.

The signal contribution of the Guide to the Expression of Uncertainty in Measurement (GUM) is the operational view of the uncertainty in measurement as a parameter, associated with a result of a measurement (measured value), that characterizes the dispersion of the values that could reasonably be attributed (assigned) to the measurand. Subsequent documents from the Joint Committee for Guides in Metrology (JCGM) have restored an essentially pre-GUM view of uncertainty and called it a coverage interval. The idea of a coverage interval requires the measurand to have a unique true value; therefore, 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 in metrology. We think that the divergence of the JCGM documents from the GUM is a consequence of misunderstanding (i) the true value and related concepts in the GUM, and (ii) the operational view of uncertainty. This paper is an attempt to correct these misunderstandings.