Examining the Impact of Drifted Polytomous Anchor Items on Test Characteristic Curve (TCC) Linking and IRT True Score Equating

Author(s):
Li, Yanmei
Publication Year:
2012
Report Number:
RR-12-09
Source:
Document Type:
Subject/Key Words:
Item response theory, item parameter drift, polytomously scored items, TCC linking, IRT true score equating

Abstract

In a common item (anchor) equating design, the common items should be evaluated for item parameter drift, and drifted items are often removed. For a test that contains mostly dichotomous items and only a small number of polytomous items, removing some drifted polytomous anchor items may result in anchor sets that no longer resemble mini-versions of the new and old test forms. In this study, the impact of drifted polytomous anchor items on the test characteristic curve (TCC) linking and IRT true score equating for a test containing only a small number of polytomous items was investigated. Simulated tests were constructed to mimic a real large scale test. The magnitude of the item parameter drift, anchor length, number of drifted polytomous items in the anchor set, and the ability distributions of the groups taking the old form and new form were manipulated. Results suggest that anchor length and number of drifted polytomous items had a relatively large impact on the linking and equating results. The accuracy of linking and equating results were affected by the magnitude of item parameter drift. The ability distributions of the groups had little effect on the linking and equating results. In general, excluding drifted polytomous anchor items resulted in an improvement in equating results.

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