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Decision rules and guard banding
Result
FAIL
Acceptance limits 99.25 to 100.75 (guard band w = 0.25) · 90.0 % of tolerance used
Same reading under the other rule: simple acceptance passes; guard band fails.
Decision rule: guard band (ILAC-G8:09/2019 §4.2.2), w = U. The acceptance limits are the tolerance limits reduced by the guard band, so a pass means the true value lies within tolerance at the stated coverage.
The method
The two rules, and what each costs.
What does simple acceptance do?
Nothing to the limits. Pass means the reading is within tolerance; the uncertainty is stated beside it and the reader draws their own conclusion about a reading near the edge. Appropriate, and common, where the test uncertainty ratio is high and the customer understands the convention. ILAC-G8:09/2019 §4.2.1.
What does a guard band do?
Moves each acceptance limit inward by w = r × U, with r = 1 as the binary case. Pass then means the true value is within tolerance at the stated coverage, not merely that the reading was. The laboratory carries the risk; the customer receives a stronger statement; instruments near the limit that would have passed now fail, in proportion to how large U is against the tolerance. ILAC-G8:09/2019 §4.2.2.
Which rule should a laboratory choose?
The laboratory's, and the standard requires only that it be chosen and stated. The test uncertainty ratio is the usual basis: at 4:1 or better simple acceptance is defensible; below it a guard band bounds a risk that would otherwise be shared silently. CalibraWorks requires the choice before it will print a pass or fail, under ISO/IEC 17025 §7.8.6, and it prints the rule by name.
What else does the certificate report?
The effective limits actually applied, so a guard band is visible rather than implied; the signed deviation from nominal; and the tolerance used as a percentage, because an instrument at 92 % of tolerance has passed and will probably fail next cycle, and a bare PASS discards that. Guidance: ILAC-G8:09/2019.
Under binary guard banding with w = U at k = 2, a reading inside the acceptance limits means the true value lies inside the tolerance with at least 97.5 % probability at each limit. Where U reaches half the tolerance, the band collapses and nothing can pass.
About this calculator
- What is a decision rule?
- A statement of how measurement uncertainty is accounted for when declaring a result pass or fail. ISO/IEC 17025 clause 7.8.6 requires it to be documented, applied, and stated on the certificate whenever a statement of conformity is made.
- What is a guard band?
- A margin, w, by which the acceptance limits are pulled inside the tolerance limits. With w = U — the expanded uncertainty at k = 2 — a reading inside the acceptance limits means the true value lies inside the tolerance with at least 97.5 % probability at each limit. That is binary guard banding in ILAC-G8 §4.2.2.
- What does simple acceptance mean?
- The measured value is compared directly with the tolerance limits and the uncertainty is not applied to them. A reading just inside the limit passes even where its uncertainty interval crosses the limit; the risk of that is shared with the customer, which is why it is also called shared risk. ILAC-G8 §4.2.1.
- Why does guard banding fail instruments that would have passed?
- Because the acceptance band is narrower than the tolerance by 2U. Where U approaches half the tolerance the band collapses and nothing can pass — the calculator says so rather than passing nothing silently. The choice between the rules distributes risk between laboratory and customer; the standard requires only that the choice be stated.
- Is this what CalibraWorks prints on a certificate?
- Yes. The engine on this page is the one that applies the rule at issue, and the sentence shown under the result is the one the certificate carries. The software refuses a statement of conformity without a declared rule.
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