Tolerance versus uncertainty
Tolerance is what you require of the instrument; uncertainty is how well the calibration could measure it. A conformity statement needs both.
A tolerance (or maximum permissible error) is a limit set by a specification or by the user. Measurement uncertainty is a property of the calibration result. They are compared, not confused: to state that an instrument is within a ±0.5 °C tolerance you need a measured error and an uncertainty small enough to make the statement meaningful.
The ratio
A common practice is to aim for a calibration uncertainty no larger than about a quarter to a third of the tolerance, so that conformity decisions are not dominated by uncertainty. Where that cannot be achieved, the decision rule must say how the risk is shared — guard banding is the usual answer.
On the certificate
ISO/IEC 17025 requires that a statement of conformity identifies the decision rule applied. Without it, 'pass' has no defined meaning.
Frequently asked questions
- What is the test uncertainty ratio (TUR)?
- The ratio of the tolerance span to the calibration uncertainty; 4:1 is a traditional target. Where it cannot be met, guard banding controls the risk of a false accept.
- Can an instrument pass if the uncertainty overlaps the tolerance limit?
- Under simple acceptance, yes, with up to 50 % chance of a wrong decision at the limit. Under a guard-band rule, no. The certificate must state which rule was applied.
References
- [1]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
- [2]ILAC-G8: Guidelines on Decision Rules and Statements of Conformity
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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What is a decision rule and why every certificate needs one
A decision rule states how measurement uncertainty is accounted for when declaring pass or fail against a tolerance. Simple acceptance, guard banding and non-binary statements are the common choices.
Measurement uncertainty explained
What an uncertainty statement means, how it is built up, and why a coverage factor matters.