Editor’s choice A
cross nearly every field of metrology, people confuse accuracy with uncertainty. It happens when measuring mass, voltage, making dimensional measurements, and based on our
own direct experience at Isotech, in temperature measurement. Very often in measurement both terms, accuracy and uncertainty, are interchanged, and thought to be the same, yet they are not. They describe fundamentally different concepts. Failing to distinguish between them can lead to significant problems, non-compliance, and costly mistakes. In temperature metrology, indeed in all measurements it is essential these terms are understood. The International Vocabulary of Metrology (VIM), published by the Joint Committee for Guides in Metrology (JCGM), exists as an international reference. The VIM defines key concepts so that scientists, regulators, and industry can communicate using a shared, consistent and technically correct language.
The VIM can be accessed online here:
www.bipm.org/en/publications/guides
WHAT THE VIM SAYS
According to the VIM (JCGM 200:2012): Definition: Accuracy (clause 2.13)
“Closeness of agreement between a measured quantity value and a true quantity value of a measurand.”
(Measurand is the quantity intended to be measured)
The VIM makes an important note here: ‘measurement accuracy’ is not a quantity, but qualitative and is not given a numerical quantity value.
Definition: Measurement uncertainty (clause 2.26)
“Non-negative parameter characterising the dispersion of the quantity values being attributed to a measurand, based on the information used.”
Uncertainty, by contrast with accuracy, is always quantitative. It defines the range within which the true value is believed to lie, expressed with a stated level of confidence.
WHY THE CONFUSION?
People often assume that if a thermometer is stated as “accurate to 0.1 °C”, the reading is correct within a tenth of a degree. In practice, that figure often only represents a design specification rather than actual verified performance. Without a properly evaluated uncertainty, such a claim is essentially meaningless.
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Accuracy describes closeness to the true value, but in practice, this “true” temperature is never perfectly known. We rely on measurement standards, which themselves have inherent tolerances, and on calibration processes that bring their own limitations.
Uncertainty quantifies the doubt remaining after all known influences have been accounted for. It describes statistically how confident we can be in the measurement result. Every measurement has uncertainty; we cannot measure without it and the VIM explicitly acknowledges this.
August 2025 Instrumentation Monthly
ACCURACY VS UNCERTAINTY: CLEARING THE CONFUSION
By David Southworth, Temperature Calibration Specialist & Technical Trainer, Isotech
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