Decision Rule Choices
ISO/IEC 17025:2017 requires that when laboratories are making statements of conformity (i.e. pass/fail or in/out of tolerance statements) to define a decision rule, apply it, and communicate it to your customer.
What is a decision rule?
Per the definition in ISO/IEC 17025:2017, a decision rule is a rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement.
ILAC has just this year published a revision to ILAC G8:2019 Guidelines on Decision Rules and Statements of Conformity. This revision clearly provides guidance on how to select appropriate decision rules and examples for the choices available based on risk.
What are the decision rule choices?
There are different options when choosing decision rules. In this article, we will be discussing the two options that we have seen used the most in our experience with laboratories.
Simple Acceptance
Most laboratories have been selecting the Simple Acceptance decision rule. The ILAC G8 essentially states (in simple language) that the specified tolerance will be used for accept or reject without factoring in the value of measurement uncertainty as long as the Test Uncertainty Ratio (TUR) is stated. Guidance is TUR of 3:1. This decision rule is considered to be a “shared risk” decision rule, because of the probability to be outside the tolerance limit may be as high as 50% when the measurement result is exactly on the tolerance limit.
Guard Banding
The simple acceptance decision rule may not be appropriate for all customers. Other decision rules are addressed and considered including Binary (limited to just pass/fail) and Non-Binary (pass, conditional pass, conditional fail, fail) Guard Banding. These decision rule techniques are designed to lower the risk of false reject and false accept caused by the relationship of tolerance and measurement uncertainty.
No matter which decision rule you decide to choose, ISO/IEC 17025:2017 para 7.1.3 requires the decision rule to be communicated to and agreed to by the customer. Remember to consider and address this requirement as you think through this decision making process.