ISO/IEC 17025:2017
Understanding Decision Rules in ISO 17025:2017
How to account for measurement uncertainty when stating conformity to a specification (Pass/Fail).
Introduction
When a customer asks you to state if a product “Passed” or “Failed” a specification, you are making a statement of conformity. ISO 17025:2017 requires you to document the “decision rule” employed, taking into account the level of risk.
The Problem: Uncertainty
Every measurement has uncertainty. If a limit is 100 and you measure 99 ± 2, is it a Pass?
- True value could be 101 (Fail).
- True value could be 97 (Pass).
Types of Decision Rules
1. Simple Acceptance (Shared Risk)
- Rule: Pass if the measured value is within the limit.
- Risk: The lab and the customer share the risk of false accept/reject.
- Usage: Common when uncertainty is small compared to the tolerance.
2. Guard Banding (Stringent Acceptance)
- Rule: Pass only if the measured value plus the uncertainty is still within the limit.
- Risk: Reduces risk of “False Accept” (consumer risk) but increases “False Reject” (producer risk).
- Usage: Critical applications (aerospace, medical).
Communicating with the Customer
You must agree on the decision rule with the customer before starting work (Clause 7.1.3). Don’t surprise them with a “Fail” due to a guard band they didn’t ask for.