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Applied Psychological Measurement
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Empirical Selection of Anchors for Tests of Differential Item Functioning

Carol M. Woods

Washington University in St. Louis, cwoods{at}artsci.wustl.edu

Differential item functioning (DIF) occurs when items on a test or questionnaire have different measurement properties for one group of people versus another, irrespective of group-mean differences on the construct. Methods for testing DIF require matching members of different groups on an estimate of the construct. Preferably, the estimate is based on a subset of group-invariant items called designated anchors. In this research, a quick and easy strategy for empirically selecting designated anchors is proposed and evaluated in simulations. Although the proposed rank-based approach is applicable to any method for DIF testing, this article focuses on likelihood-ratio (LR) comparisons between nested two-group item response models. The rank-based strategy frequently identified a group-invariant designated anchor set that produced more accurate LR test results than those using all other items as anchors. Group-invariant anchors were more difficult to identify as the percentage of differentially functioning items increased. Advice for practitioners is offered.

Key Words: differential item functioning • item bias • measurement invariance • item response theory • likelihood ratio

This version was published on January 1, 2009

Applied Psychological Measurement, Vol. 33, No. 1, 42-57 (2009)
DOI: 10.1177/0146621607314044
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