Decision Rules

Decision rules define the criteria and methodology for declaring whether an instrument conforms to its tolerance, taking measurement uncertainty into account. ISO/IEC 17025:2017 requires that calibration laboratories agree on decision rules with their customers when conformity statements (pass/fail declarations) are requested, and that these rules account for the risk of false decisions due to measurement uncertainty.

There are several common approaches to decision rules. Simple acceptance uses the tolerance limits as-is, accepting the risk that measurement uncertainty may cause false accepts. Guard-banded acceptance narrows the acceptance limits to control false accept risk. Binary conformance (ASME B89.7.3.1) considers the instrument "in tolerance" only if the measured value plus the expanded uncertainty falls within tolerance. Shared-risk acceptance places the tolerance limit at the boundary, with the customer accepting the risk of false decisions.

For calibration management, decision rules are a critical component of calibration procedures and certificates. The decision rule used must be documented and agreed upon with the customer before calibration. Different decision rules are appropriate for different situations: high-consequence measurements (safety, regulatory) warrant conservative rules with guard banding, while lower-risk applications may accept shared-risk approaches. Calibration software should support multiple decision rule options and clearly indicate which rule was applied on each certificate.