equipercentile

Equipercentile is a method used in test development and analysis to create a score linkage between two tests or forms by aligning their percentile ranks. The central idea is that a score on one test that corresponds to a given percentile should be equated to the score on the other test that has the same percentile. The resulting crosswalk allows scores from different assessments to be interpreted on a common scale, reflecting how examinees of the same relative standing would perform on each test.

The typical process involves constructing percentile curves for each test from a reference sample. For each percentile, the corresponding score on Test A and the corresponding score on Test B are identified, and a mapping is created between the two scores. In practice, interpolation is often used to fill gaps for percentiles not directly observed, and smoothing procedures may be applied to reduce sampling noise.

Equipercentile equating makes no assumption about equal score intervals or distributions between tests; instead, it preserves relative standing within each test’s distribution. It is especially useful when tests measure the same or closely related constructs but have different formats, content, or scales. However, the method relies on the comparability of the populations used to construct the percentile curves; changes in ability distributions or demographics between samples can affect the accuracy of the linking.

Applications of equipercentile linking include equating different test forms, standardizing scores across remotely administered assessments, and enabling fair score interpretation in retention or certification programs. Limitations include sensitivity to sample size and distribution differences, and potential instability when distributions are markedly non-overlapping or when security or practice effects alter percentiles over time. See also test equating, score linking.