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A Didactic Approach to the Use of IRT True-Score Equating IRT 3PL

von Davier, Alina A.; Wilson, Christine
Publication Year:
Report Number:
ETS Research Report
Document Type:
Page Count:
Subject/Key Words:
Population Sensitivity, Test Equating, Item Response Theory (IRT), True-Score Equating Method, Observed-Score Equating, 3-parameter logistic (3PL)


This paper discusses the assumptions required by the item response theory (IRT) true-score equating method (with Stocking & Lord, 1983; scaling approach), which is commonly used in the nonequivalent groups with an anchor data-collection design. More precisely, this paper investigates the assumptions made at each step by the IRT approach to calibrating items and equating tests, and discusses the approaches that one might take for checking whether these assumptions are met for a particular data set. We investigated two types of tests: tests that consist of multiple-choice items only, and tests that consist of both multiple-choice and free-response items. Real data from the AP® Calculus AB exam are used to illustrate the application of the IRT true-score equating method as well as for the comparisons.

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