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Determining the Anchor Composition for a Mixed-Format Test: Evaluation of Subpopulation Invariance of Linking Functions

This study examined the appropriateness of the anchor composition in a mixed-format test, which includes both multiple-choice (MC) and constructed-response (CR) items, using subpopulation invariance indices. Linking functions were derived in the nonequivalent groups with anchor test (NEAT) design using two types of anchor sets: (a) MC only and (b) a mix of MC and CR. In each anchor condition, the linking functions were also derived separately for males and females, and those subpopulation functions were compared to the total group function. In the MC-only condition, the difference between the subpopulation functions and the total group function was not trivial in a score region that included cut scores, leading to inconsistent pass/fail decisions for low-performing examinees in particular. Overall, the mixed anchor was a better choice than the MC-only anchor to achieve subpopulation invariance between males and females. The research reinforces subpopulation invariance indices as a means of determining the adequacy of the anchor.