Tolerances and decision rules explained
A tolerance states the permitted limits for a value or error. A decision rule states how a measurement result and its uncertainty are used to decide whether the requirement is met. The resulting statement of conformity is the reported conclusion, such as pass or fail.
Decide which specification and rule apply before judging a result. The method may come from a specification, agreed customer requirement or your controlled procedure. UKAS LAB 48: Decision rules and statements of conformity explains simple acceptance and how guard bands change the acceptance interval and decision risk.
Tolerance is different from uncertainty
Suppose a thermometer's permitted indication error is -0.50 to +0.50 °C. That tolerance is a requirement. The uncertainty describes the measurement used to assess the error; a smaller uncertainty does not change the specified tolerance.
For an introduction to those measurement terms, read Measurement uncertainty explained.
A worked example of two Metra rules
Consider an error of +0.43 °C and expanded uncertainty of 0.10 °C. Assume the boundaries are included in this example.
| Selected rule | Acceptance interval | Outcome for +0.43 °C |
|---|---|---|
| Simple acceptance | -0.50 to +0.50 °C | Pass: the error is inside the interval. |
| Fixed guard band of 0.10 °C | -0.40 to +0.40 °C | Fail under this binary rule: the error is outside the acceptance interval. |
A guard band is the margin between a tolerance limit and an acceptance limit. Here it narrows the interval for acceptance. The measured error is unchanged; the controlled acceptance policy differs. This example illustrates Metra's supported rules, not a rule that every laboratory should adopt.
Using an uncertainty-based guard band is another option in Metra. Its revision specifies the selected uncertainty type and multiplier. Do not substitute expanded uncertainty for standard uncertainty, or choose a different rule after seeing an inconvenient outcome.
What the result labels mean
| Label | How to read it |
|---|---|
| Pass | The applicable rule's acceptance conditions were met. |
| Fail | The applicable rule's acceptance conditions were not met. Review the rationale and limits. |
| Indeterminate | The applicable process has not established a definitive pass or fail. It needs review. |
| Not evaluated | No completed conformity evaluation is available for the point. |
An uncertainty interval overlapping a tolerance limit does not automatically mean one particular status. The declared decision rule determines how that situation is handled. Read the result's rationale rather than applying a different rule mentally.
How Metra keeps the decision explainable
Metra retains the selected rule revision, tolerance, measured error, selected uncertainty, acceptance interval and decision evidence. It calculates the outcome on the backend; displayed rounding should not be used to recalculate a borderline result.
A point's conformity decision is separate from the overall calibration review, accreditation eligibility and the equipment's next due date. A failed or indeterminate calibration may also require an out-of-tolerance investigation.