Decision Rules in Calibration and Testing: How Measurement Results Become Conformity Decisions

A measurement result does not automatically show whether an item conforms to a specification. The result must be compared with a tolerance, and measurement uncertainty must be considered. A decision rule explains how this comparison will be made. Different rules can produce different conformity statements from the same result, especially near a tolerance limit. This working paper explains decision rules in plain language. It introduces simple acceptance, guard bands, false acceptance, false rejection, and shared risk. It also describes the requirements of ISO/IEC 17025 and gives practical examples from calibration and medical metrology. The main conclusion is that a conformity statement is not only a technical calculation. It is a documented risk decision that should be understood and agreed before the work begins.