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True value and uncertainty in measurement



Raghu N. Kacker


We will discuss the concept of true value and its connection with the uncertainty in measurement 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, JCGM 104, and JCGM 200. Most measurands of interest in metrology are ordinary measurands – that are specified by a description. The description of an ordinary measurand is necessarily incomplete because only significant measurand-affecting quantities are specified. Therefore, every ordinary measurand has a range of true values. The concept of uncertainty in measurement established by the GUM is concerned only with the values assigned to the measurand based on measurement. Since uncertainty in measurement does not refer to the concept of true value, 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 with a stated coverage probability does not apply to an ordinary measurand that has a range of true values. When it applies, an evaluated coverage probability is never strictly correct.


BIPM, CIPM, Coverage interval, Coverage probability, GUM, GUM-S1, JCGM, Measurement, True value, Uncertainty, VIM1, VIM2, VIM3


Kacker, R. (2018), True value and uncertainty in measurement, Measurement, [online], (Accessed June 17, 2024)


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Created September 2, 2018, Updated September 18, 2020