Reporting results and statements of conformity (clause 7.8)

Clause 7.8 sets out the required content of reports and certificates. For calibrations it adds the measurement uncertainty presented in the same unit as the measurand or relative to it, the conditions under which the calibration was made, a statement of metrological traceability, and results before and after any adjustment or repair.

Decision rules

Where a statement of conformity to a specification is made, the laboratory must document the decision rule employed and take account of the risk associated with it — false accept and false reject — and apply it as agreed with the customer. ILAC-G8 describes common rules: simple acceptance (uncertainty ignored, shared risk), guard banding (acceptance limits narrowed by the uncertainty or a fraction of it), and non-binary statements such as 'conditional pass'.

What clause 7.8.2 requires on every report

The standard lists the minimum content, and the list is short enough that omissions are usually oversights rather than judgement calls. A report has to identify the laboratory and the customer, the items calibrated or tested and when they were received, the method used, the date of performance and the date of issue, the results with their units, and the name of the person authorising the report. It must also say clearly where any part of the work was subcontracted.

For calibration certificates, clause 7.8.4 adds the measurement uncertainty in the same unit as the measurand or relative to it, the conditions under which the calibration was made, evidence that results are metrologically traceable, and — where an instrument was adjusted or repaired — the results both before and after. That last requirement is the one most often missed, and it is the one a customer most needs: an as-found reading is the only evidence of whether the instrument was fit for the work it did since its last calibration.

A conformity statement, worked through

Suppose a logger is checked against a ± 0.10 °C specification. The measured error is 0.08 °C and the expanded uncertainty is U = 0.05 °C at k = 2. The measurement interval runs from 0.03 °C to 0.13 °C and crosses the limit. Every one of the following is a defensible certificate entry, and they say different things:

None of these is the right answer in the abstract. The right answer is the one agreed with the customer before the work, documented, and named on the certificate. A bare pass with no rule stated is the one option the standard does not allow, because the reader cannot tell which of the three rows above produced it.

Opinions and interpretations

Clause 7.8.7 treats an opinion or interpretation as a distinct thing from a result. If the certificate says the drift observed suggests shortening the calibration interval, that is an opinion: it must be identified as such, the basis for it documented, and the personnel authorised to issue it defined. Mixing an opinion into the results section, unlabelled, is a common nonconformity.

The findings assessors raise most often

• A pass or fail statement with no decision rule named anywhere on the certificate.
• Uncertainty quoted without its coverage factor or coverage probability, so the number cannot be interpreted.
• As-left results only, with no as-found values after an adjustment.
• A traceability statement that names a laboratory but not the standards, or cites an accreditation number that has since lapsed.
• Amended reports issued without a unique identifier or a reference to the version they replace.