= Proficiency testing
\b[Proficiency testing] evaluates participants' performance against established criteria through an <interlaboratory comparison>. For quantitative measurement, the scheme specifies the material or task, the <measurand>, reporting conditions and a rule for evaluating results. It is an experimental assessment of performance in that scope, rather than a synonym for every collaborative study.<Selection, Use and Interpretation of Proficiency Testing (PT) Schemes by Brian Brookman and Ian Mann>[§3, printed p. 4, definition; §§4.1–4.2, role and scheme types]{ref}
A typical round distributes comparable <proficiency-test items> to laboratories, which analyse them using their routine <measurement procedures>. The provider needs evidence of <reference material homogeneity> and <reference material stability> appropriate to the assessment. Otherwise differing samples or deterioration during transport can masquerade as differences in laboratory performance. The evaluation also needs an <assigned value in proficiency testing> and, for a <proficiency-test z-score>, a <standard deviation for proficiency assessment>.<Selection, Use and Interpretation of Proficiency Testing (PT) Schemes by Brian Brookman and Ian Mann>[§§7.2.1–7.2.4 and Annex D, assigned values, uncertainty and scoring]{ref}
An original two-level experiment distinguishes possible response patterns. Suppose independent assigned values are $10$ and $20$ in the same units. One laboratory reports $11$ and $22$; another reports $11$ and $21$. The first pattern fits a multiplicative factor of 1.10 at both levels, whereas the second fits an additive offset of 1. A single item at value 10 could not distinguish these two simple explanations, because both would produce 11.
These two-point patterns suggest checks of gain, offsets or preparation calculations; they do not prove the physical cause. Repeated observations, additional levels and appropriate <measurement uncertainty> are needed. If the scheme uses assessment scale 0.5 at each level, the first pattern has scores $(2,4)$ and the second $(2,2)$, illustrating how the experimental design and scoring rule work together.
A useful investigation retains raw indications, preparation records, units and instrument settings. Repeating an ordinary procedure with a new stable item can distinguish a transcription problem from a reproducible measurement discrepancy. Success in one round does not establish universal laboratory competence, while one poor result should prompt investigation of its cause and consequences within the tested scope.<Selection, Use and Interpretation of Proficiency Testing (PT) Schemes by Brian Brookman and Ian Mann>[§8.2, laboratory evaluation; §8.3, investigating unsatisfactory performance]{ref}
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