 Proficiency-test item

A proficiency-test item is the material, artefact, instrument or information supplied for assessment in a proficiency testing scheme. It is the object to which a specified task and result refer; it is not itself the participant's reported result or the assigned value in proficiency testing. The primary guide's definition covers samples and physical standards as well as data or other information used for assessment.^[ref]

For a material example, the provider can prepare a batch, divide it into sealed units, and distribute one coded unit to each laboratory. The specified measurand, amount to be sampled, preparation instructions, reporting units and handling conditions belong to the comparison design. The material must support the particular measurement procedures being assessed, rather than merely have a convenient nominal concentration.

Reference material homogeneity and reference material stability protect the interpretation. An original counterexample gives two laboratories bottles whose actual mass concentrations are $8$ and $12mg/L$, while both are scored against $10mg/L$ with assessment scale $0.5mg/L$. Perfect measurement of each bottle produces proficiency-test z-scores of $-4$ and $+4$. Those discrepancies arise from unequal items, not from incorrect laboratory measurement. A perfectly stable material can still be nonuniform, and a uniform batch can still change during transport.

A simple item heterogeneity contribution to proficiency-test scores can be written in a measurement model,where ${\delta }_{j(i)}$ describes the supplied unit's deviation, $b_i$ the laboratory effect and ${\epsilon }_i$ residual variation. Observing $x_i$ alone cannot assign all of its difference from $\theta$ to $b_i$. Replicate portions from selected units and stability checks across the relevant interval supply additional evidence.

The required degree of item uniformity depends on the assessment criterion. Narrowing the standard deviation for proficiency assessment can make previously tolerable item differences significant. The guide consequently requires checking whether item homogeneity and stability remain appropriate when choosing a stricter assessment scale.^[ref]

When a single artefact is circulated sequentially, changes between visits can similarly be confounded with laboratory differences. Record the item identifier, its preparation or transport history and the actual observation conditions so the reported value refers to a reproducible task.