Decision rule
Explanation of the use of the decision rule in accordance to DIN EN ISO/IEC 17025:2018
Dear customer,
According to the standard DIN EN ISO/IEC 17025:2018 (General requirements for the competence of testing and calibration laboratories), there is a requirement to document decision rules that are made during a conformity assessment of test results.
Decision rules are rules that describe how measurement uncertainty consider when making statements about conformity with a specified requirement.
The selection of the decision rule follows the following scheme
- Is a declaration of conformity with a metrological specification required? If Yes, then continue with 2), if No, no conformity assessment takes place.
- If the measurement result is regulated by legal or regulatory standards or rules, the decision rule as defined in the corresponding standard is used. e.g. VLOG standard. If No, then continue with 3)
- The application already contains measurement decision rules that are included in a published standard guide. In these cases, they are standard test methods that already have compliance limits built in. e.g. Drinking Water Regulation. If no, then continue with 4)
- If none of the preceding cases in items 2 or 3 occur, this generally means that the evaluation is not driven by any published decision rule. In this case, the following decision rules are applied.
Case differentiation
1) Measured value is outside the limit, measurement uncertainty is outside the limit.
- Conformity statement: Limit/specification is exceeded, the specimen is not conforming. The risk of an incorrect evaluation is very low, because the expanded measurement uncertainty (k=2) is usually included in the decision.
2) Measured value is outside limit, measurement uncertainty is within limit
- Conformity statement: The measured value is above the limit value. After deduction of the expanded measurement uncertainty (k=2), the content still meets the requirements. The sample is assessed as conforming. Taking into account the expanded measurement uncertainty, however, the content could also not meet the requirements. An exceedance can therefore not be excluded with certainty.
3) Measured value is within limit, measurement uncertainty is outside limit
- Conformity statement: The measured value is below the limit value and the sample is assessed as conforming. Taking into account the expanded measurement uncertainty (k=2), however, the content may not meet the requirements. An exceedance and thus a complaint cannot be excluded with certainty.
Measured value is within limit, measurement uncertainty is within limit
- Conformity statement: Limit value/specification is undercut, specimen is in conformity. The risk of an incorrect evaluation is very low, because the expanded measurement uncertainty (k=2) is usually included in the decision.
In microbiological analyses, the measurement uncertainty is generally taken into account in limit values. The following statements on conformity follow from this:
Case 1) and 2) non-conforming; Case 3) and 4) conforming