Conformance Decision Rules to Support ISO/IEC CD2 17025 Under Revision

Abstract

The current draft of ISO/IEC CD 17025 contains a clause to document the decision rule for conformity along with the risk and statistical assumptions in the test method or procedure (7.1.1d). So how shall we do this? A common strategy for managing measurement decision risk is to choose a guard-band that results in a desired false-accept risk given a tolerance limit, the calibration process uncertainty, and the a priori probability. JCGM 106:2012 provides guidance for the implementation of prior knowledge in conformance decisions, however a good estimate of the a priori probability may be difficult to obtain. Historical device population information for estimating a priori probability may not be readily available such as "testing" applications where the device is a prototype. This paper presents the false-accept risk and corresponding false-reject risk for four decision rules. We provide simple equations to limit risk to pre-determined amounts. Several case studies illustrate when to use "Specific" or "Global" risk. When "Global" risk is appropriate, the decision rules can be applied with limited knowledge of a priori probability.