Are statements of conformity (and, indirectly, decision rules) applicable to non-dichotomic cases?
For example, in situations where a sample may be classified in several classes (A, B, C, D), depending on the testing results, should a decision rule be established and applied when categorizing a specific sample in a specific class?
Background:
ISO/IEC 17025 includes an acceptable definition for decision rule (rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement), although ambiguous regarding the possibility of establishing that uncertainty can be took into account by stating that it is to be ignored – this should also be harmonized at EA level.
However, there is no Statement of conformity definition.
ISO/IEC 17025 weakly introduces the concept of Statement of conformity (7.1.3 When the customer requests a statement of conformity to a specification or standard for the test or calibration (e.g. pass/fail, in-tolerance/out-of-tolerance) …), and the best that we can find in the CASCO toolbox comes from ISO/IEC 17000 itself, namely:
7.3 attestation issue of a statement, based on a decision (7.2), that fulfilment of specified requirements (5.1) has been demonstrated
Note 1 to entry: The resulting statement, referred to in this document as a “statement of conformity”, is intended to convey the assurance that the specified requirements have been fulfilled. Such an assurance does not, of itself, provide contractual or other legal guarantees.
A.4.1 “Review” (see 7.1) constitutes the final stage of checking before taking the important “decision” (see 7.2) as to whether or not the object of conformity assessment has been reliably demonstrated to fulfil the specified requirements. “Attestation” (see 7.3) results in a “statement” in a form that most readily reaches all of the potential users. “Statement of conformity” is a generic expression used to include all means of communicating that fulfilment of specified requirements has been demonstrated.
It can be argued, both from the examples in ISO/IEC 17025 and from the wording in ISO/IEC 17020, that the idea behind the concept of Statement of conformity is that either the fulfillment of specified requirements has been demonstrated or not (or that nothing can be affirmed) and that this is not the case when you classify an object of conformity assessment according to multiple options.
However, from a formal logic perspective, multiple options can be reduced to several nested Yes/No decisions.
Non-dichotomic cases as described in the question can be considerd a kind of classification as results on the tested items are compared with the limits defining the different classes. This means that the attribution of the class can be considerd as a statement of conformity and therefore the appropriate aspects of ISO 17025:2017 to be considered.
Where further inferences are made by including knowledge and professional judgement of a technically qualified person, then the laboratory declaration should be considerd as an Opinion and Interpretation.