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.
A logger reads 0.08 °C high against a tolerance of ±0.10 °C, with U = 0.05 °C. Is it in tolerance? The interval 0.03–0.13 °C crosses the limit. A decision rule is the agreed answer to that question.
Common rules
- Simple acceptance (shared risk): pass if the measured value is within tolerance, uncertainty ignored. Probability of false accept at the limit is up to 50 %.
- Guard band with w = U: pass only if the value is within tolerance minus U. Probability of false accept ≤ about 2.5 %.
- Guard band with w = U/2 or another fraction: a negotiated intermediate.
- Non-binary: 'pass', 'conditional pass', 'conditional fail', 'fail' bands that state where the interval lies.
ILAC-G8 and JCGM 106 describe these rules and the risks they imply. The rule should be chosen for the consequence of a wrong decision, then written down before the measurement.
Frequently asked questions
- Which rule does a typical calibration laboratory use by default?
- Many state simple acceptance unless the customer specifies otherwise, and say so on the certificate. Regulated customers often require a guard band.
- Does the rule change the measured value?
- No. It changes only the conformity statement. The value and uncertainty are reported unchanged.
References
- [1]ILAC-G8: Guidelines on Decision Rules and Statements of Conformity
- [2]JCGM 106:2012 — The role of measurement uncertainty in conformity assessment
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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