IRT Ability Estimates from Customized Achievement Tests Without Representative Content Sampling

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Abstract
This study examines the effects of using item response theory (IRT) ability estimates based on customized tests that were formed by selecting specific content areas from a nationally standardized achievement test. Subsets of items were selected from four different subtests of the Iowa Tests of Basic Skills (Hieronymus, Hoover, & Lindquist, 1985) on the basis of (a) selected content areas (content-customized tests) and (b) a representative sampling of content areas (representative-customized tests). For three of the four tests examined, ability estimates and estimated national percentile ranks based on the content-customized tests in school samples tended to be systematically higher than those based on the full tests. The results of the study suggested that for certain populations, IRT ability estimates and corresponding normative scores on content-customized versions of standardized achievement tests cannot be expected to be equivalent to scores based on the full-length tests.

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