A properly developed consensus from a proficiency test is, for all practical purposes, interchangeable with a certified value for a matrix reference material derived from an interlaboratory comparison

A properly developed consensus from a proficiency test is, for all practical purposes, interchangeable with a certified value derived from an interlaboratory comparison. But some scientists have doubts about the metrological status of such values. In particular, the materials are not regarded as equivalent to certified reference materials (CRM) because of a lack of traceability of the consensus value. However, the detailed considerations in this study show that such a distinction is without foundation. The following issues are addressed. (a) Misgivings about the traceability of an interlaboratory consensus are found to be insubstantial because chemical measurements are hardly ever fully traceable. (b) Reproducibility (i.e., interlaboratory) conditions account for all sources of variation in results other than long-term stability, so lead to a valid estimate of uncertainty. (c) Certification of a reference material according to ISO procedures is homologous with a proficiency test and no more secure against bias in the characterised value and its standard error. In particular, residual (common) bias cannot be fully addressed in either approach. (d) A consensus in itself attests to a valid outcome. (e) Proficiency test providers have access to a powerful method of monitoring long-term stability that is denied to CRM producers

Dark uncertainty

Standard uncertainties obtained by the GUM approach for a range of analytical methods are compared directly and indirectly with estimates of reproducibility standard deviation for the same methods. Results were obtained from both routine analysis and international key comparisons. A general tendency for the uncertainty to be substantially less than the reproducibility standard deviation was found