Decision Rules -Reporting Conformity
Updated: May 26, 2022
ISO 17025:2017 requires that a decision rule should be applied to any statement of conformity to a specification or standard in a test report when it is requested by customers. The decision rule applied should also take into account the level of risk associated with the decision, primarily in terms of an incorrect decision. Measurement uncertainty should be incorporated in any decision rule applied to the reported result.
Reporting Conformity : Decision Rules
ISO 17025:2017 requires that a decision rule should be applied to any statement of conformity to a specification or standard in a test report when it is requested by customers. The decision rule applied should also takes into account the level of risk associated with the decision, primarily in term of incorrect decision. Measurement uncertainty should be incorporated in any decision rule applied to the reported result.
Definition of Decision Rule
Rule that describes how measurement uncertainty is accounted for when stating conformity with specified requirement.
SCR laboratory lists 6 (six) possible decision rules will be applied to result when customer request for statement of conformity in compliance to a specification or the test inherently requires statement of conformity such as in compliance to regulatory standards or limits. For compliance to customer specification, customer may decide on which decision rules to be applied while for regulatory standard compliance, Simple Acceptance Rule #1 will be applied if not directive is prescribed in the standard guideline or procedures.
The following decision rules may be applied to test result when statement of conformity is required. Detail descriptions of the decision rule and associated risk to incorrect decision are shown below.
1. Decision Rule #1 : Simple Acceptance with application of measurement uncertainty
If measurement result ± uncertainty of measurement falls within the upper and lower tolerance limit, it is considered compliance. This rule means that when test result minus the measurement uncertainty becomes lower than the upper tolerance limit, the result is considered comply or pass. However, for this case, the risk to incorrect decision is high. In order to apply this decision rule, the Test Uncertainty Ratio (TUR) must be greater than 3:1 which means that the expanded uncertainty (U) at the tolerance limit must be less than 33% of the limit value.
2. Decision Rule #1A : Simple Acceptance without applying measurement uncertainty
It is allowed but not encouraged that customer choose this decision rule which is applied without considering the measurement uncertainty. However, MU at the result will be included in the report. Customer may use the MU to determine the conformity and risk.
Acceptance is when result is within the lower and upper tolerance limit, and rejection or non-compliance is when result is outside the tolerance interval. Risk to incorrect decision will not be provided.
3. Decision Rule #2 – Setting compliance probability at 95%
When compliance probability is specified, decision rule should comply with these requirement,
Acceptance if H0: P(y ≤ Tu) ≥ (1-a) is true.
Rejection if H0 is false i.e. P(y ≤ Tu ) < (1- a)
This decision rule may be strict or relax depending on the required probability of compliance (PC). Decision Rule #2 is applied by setting the PC to 95% which means that the error to incorrect decision is limited to 5% only. If this decision rule is chosen by customer, SCR will help to calculate the acceptance limit to be applied.
The probability of conformance (PC) may be calculated using Microsoft Excel function
NORM.DIST (x, mean, SD, Cumulative)
where x = measurement result, mean = acceptance limit, SD = measurement uncertainty and Cumulative is set as TRUE.
If this greater than or equal to 0.95, then the result is considered comply or pass. Otherwise it is non-compliance or fail.
4. Decision Rule #3GA – Guarded Acceptance
This decision rule is made after adding a guarded acceptance band inside the acceptance zone. This will reduce the false acceptance but may increase the probability of incorrect decision.
When the guard band width is set as equal to the expanded uncertainty (U), this decision rule #3GA is acceptable by the ILAC-G8:09/2019 guideline (Choice B in the flow-chart).
Customer may opt for this decision rule rather than the simple acceptance Decision Rule #1
Another option is to set the guard band width such that the decision will produce less than 2% probability of false acceptance (PFA). SCR will assist customer to determine the guard band width which will result in PFA of 2% or less if you choose this decision rule.
5. Decision Rule #3GR – Guarded Rejection
This Decision Rule is made after adding a guarded acceptance band outside the acceptance zone. This will reduce the false rejection but may increase the risk to incorrect decision.
This is a more lenient acceptance rule compared to Decision Rule #3GA.
When the guard band width is set as equal to the expanded uncertainty (U), this decision rule #3GR is acceptable by the ILAC-G8:09/2019 guideline (Choice B in the flow-chart).
This decision rule may be adopted if customer want to be sure the test result is really outside the acceptance limit before it is rejected.
6. Decision Rule #X – Customer’s Specification
Customer may set the acceptance limits/zone according to their own consideration. This decision rule should not be applied to regulatory compliance even when not guideline is given. Technically, all result may be set to comply if the acceptance limit is purposely set above the tolerance limit.
Compliance statement will state that this is from customer’s specification.
If the measurement uncertainty is to be used, customer may specify that the acceptance limit should be in a multiple of standard uncertainty.
When the acceptance limit is set at the expanded uncertainty, the decision rule is similar to Decision Rule #3GA or #3GR depending on where the guard band is applied.
SCR may recommend the use of the other decision rules available which can be modified using the customer’s specifications.