A decision rule describes how measurement uncertainty is accounted for when stating conformity to a specification. ISO/IEC 17025:2017 Clause 7.1.3 requires the laboratory to agree and document the decision rule whenever it issues statements of conformity, and ILAC G8:2019 provides the framework. Guard banding is the most widely used decision rule: instead of accepting any result that falls within the tolerance, you accept only results that fall inside the tolerance by a guard band — usually derived from the measurement uncertainty — which reduces the probability of falsely accepting a nonconforming item. Without an agreed decision rule, a pass/fail statement near the tolerance limit is not defensible, because the measurement could be on either side of the true limit.
Every measured value carries uncertainty, so a reading sitting right at the tolerance limit could correspond to a truly conforming or a truly nonconforming item. Simple acceptance — accepting any result within the tolerance, with a guard band of zero — leaves a probability of false acceptance approaching 50% for items whose true value is exactly at the limit. Guard banding deliberately trades a few more false rejections for far fewer false acceptances, which is the right trade when the cost of shipping a bad part (a flight-critical dimension, a medical-device characteristic) dwarfs the cost of re-working a marginal good one.
Guard banding shrinks the acceptance zone inward from each tolerance limit by a guard band g, so the acceptance limit becomes the tolerance limit minus g. A common stringent choice sets g equal to the expanded uncertainty U (coverage factor k = 2), which targets a probability of false acceptance around 2.5%; a more general rule sets g = w·U, where w is chosen for the desired risk. This creates three zones: an acceptance zone (inside the guarded limit), a guard band where the result is too close to the limit to claim conformity with confidence, and a rejection zone beyond the tolerance. A related rule of thumb is the test uncertainty ratio (TUR): when the TUR is 4:1 or better — the tolerance is at least four times the expanded uncertainty — the guard band is small relative to the tolerance and simple acceptance is often considered acceptable by agreement.
ILAC G8 frames decision rules as a spectrum. Binary simple acceptance (guard band of zero) accepts anything within tolerance and accepts the associated risk. Binary guarded acceptance or guarded rejection moves the decision limit inward or outward by a guard band to control false-accept or false-reject risk. Non-binary rules add an explicit conditional or indeterminate zone where no conformity statement is made. Shared-risk rules are agreed with the customer where both parties accept the consumer/producer risk. The essential point of Clause 7.1.3 is that whichever rule applies must be agreed with the customer and documented before conformity is stated.
Suppose a characteristic has a tolerance of plus or minus 0.50 and the calibration has an expanded uncertainty U of 0.10 (k = 2). Under simple acceptance, you accept any measured error with magnitude up to 0.50. Under guarded acceptance with a guard band equal to U, the acceptance limit moves inward to 0.40, and the 0.40-to-0.50 band becomes a conditional zone where conformity cannot be claimed. A reading of 0.45 therefore passes under simple acceptance but does not qualify for an acceptance statement under the guarded rule — the same number, a different and more defensible decision. Stating which rule was used is what makes the resulting certificate withstand scrutiny.
CalibrationOS lets you configure decision rules and guard bands and ties them to the calibration's uncertainty budget, so the probability of false acceptance is controlled rather than assumed. Results that fall inside the guard band are flagged rather than silently passed, and the applied decision rule is recorded with the result — satisfying the Clause 7.1.3 requirement to agree and document the rule behind every conformity statement.
A decision rule describes how measurement uncertainty is accounted for when stating conformity to a specification. ISO/IEC 17025:2017 Clause 7.1.3 requires the laboratory to agree and document the decision rule whenever it issues conformity statements.
Guard banding is a decision rule that accepts a result only if it falls inside the tolerance by a guard band, usually derived from the expanded measurement uncertainty. It reduces the probability of falsely accepting a nonconforming item near the tolerance limit.
ILAC G8:2019 provides the framework for decision rules and conformity statements. It describes binary, guarded, non-binary, and shared-risk rules, and requires that the chosen rule be agreed with the customer and documented before a statement of conformity is made.
A common stringent choice sets the guard band equal to the expanded uncertainty U (k = 2), targeting roughly a 2.5% probability of false acceptance. More generally the guard band is g = w·U, with w chosen for the desired risk and agreed with the customer.
The test uncertainty ratio (TUR) compares the tolerance to the expanded measurement uncertainty. When the TUR is 4:1 or better, the uncertainty is small relative to the tolerance and simple acceptance (no guard band) is often considered acceptable by agreement.
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