Risks and the consequences of incorrect decision-making in conformity assessment can be minimised by setting limits on maximum permissible measurement uncertainties and on maximum permissible consequence costs at the specification stage of any task. After that, one can also agree on acceptable locations of the uncertainty interval with respect to a specification limit, as follows.
It is emphasised that – apart from the following default rules for proving conformance and non-conformance to specification – both supplier and customer may reach agreement about other rules. In some sectors, such as GPS, in cases where conformance to the most important specifications are being assessed, it is recommended that the default GPS rules shall always be applied [ISO 14253-1:1998 §5.1].
The specifications on the quality characteristic of the entity subject to conformity assessment are of course fixed and invariable throughout a particular manufacturing task. This means that the locations are fixed of the upper (USL) and lower (LSL) specification limits with which the region of permissible values is defined. (There is no “specification uncertainty” [ISO/TS 17450-2].)
On the other hand, each measurement made during manufacturing or subsequent verification will have variable measurement quality so that the measurement uncertainty may vary from measurement to measurement.
Decision rules in a supplier/customer relationship
According to the GPS approach, a test result which lies within the guard-band zone at each specification limit results in an indeterminate decision of compliance. Neither conformance nor non-conformance can be proved in this case. [§5.4 of ISO 14253-1:1998]. Consequently, workpieces or measurement equipment cannot be accepted or rejected here.
This will be a dilemma which needs to be resolved in any supplier/customer relationship.
If no previous agreement has been reached between supplier and customer, the GPS rules are based on the principle that the “uncertainty of measurement always counts against the party who is providing the proof of conformance or non-conformance and therefore making the measurement” [§6.1 of ISO 14253-1:1998]:
- Supplier proving conformance with his estimated measurement uncertainty and guardband equation A7.1
- Customer proving non-conformance with his estimated measurement uncertainty and guardband equation A7.2
- ISO 14253-1:1998, Geometrical Product Specification (GPS) — Inspection by measurement of workpieces and measuring instruments — Part 1: Decision rules for proving conformance or non-conformance with specifications.
- Conformity assessment:
- Specifications of Process and Measurement Capabilities
- Risks for incorrect decisions
- Customer and suppler risks
- Introducing impact and cost into conformity assessment risks