The ‘Decision Rule’ requirements are new addition to the ISO 17025:2017 standard for Laboratory Quality Management, but how should you implement it?
Where a statement of conformity has a risk of false accept or false reject the basis for the statement, the decision rule, shall be agreed with the customer and reported (unless the rule is prescribed by the standard test method). As such, it is recommended that test laboratory establish a default policy or each test method documents the decision rule to be adopted.
The intended ‘decision rule’ should be communicated to the customer when establishing contracts and varied by the customer as required (forming a variation of the default decision rule) which would also require documenting contractually and in the output reports / certificates.
ILAC G8:03 2009 Guidelines on the Reporting of Compliance with Specification used to suggest: “In general the coverage probability will be 95 % and the reporting shall include a remark such as “The statement of compliance is based on a 95% coverage probability for the expanded uncertainty.” This means that the probability that the measurement is below the upper specification limit is higher than 95 %, i.e. approximately 97.5 % for symmetrical distributions. A lower limit is treated similarly.” However ILAC G8 was reviewed in September 2019 and, where nothing is prescribed, it now puts more control in the hands of the laboratories and their customers to decide how to manage false accepts and rejects.