Detecting Differential Item Functioning of Polytomous Items for an Ideal Point Response Process

Wei Wang, Louis Tay, Fritz Drasgow

Research output: Contribution to journalArticlepeer-review


There has been growing use of ideal point models to develop scales measuring important psychological constructs. For meaningful comparisons across groups, it is important to identify items on such scales that exhibit differential item functioning (DIF). In this study, the authors examined several methods for assessing DIF on polytomous items generated by an ideal point process. Two paradigms (i.e., null hypothesis significance testing [NHST] and effect size quantification) were utilized, and three test statistics (i.e., the log-likelihood ratio [LR], the Akaike information criterion [AIC], and Lord's chi-square) and two approaches to DIF testing (i.e., the constrained and free baseline methods) were evaluated. In addition, the authors investigated three levels of impact. The results revealed that DIF effect sizes were moderately large for the.50 uniform DIF conditions and small for nonuniform DIF; moreover, the LR test in general yielded the best results. When there was small to moderate impact, the free baseline approach combined with an item linking implementation produced the most satisfactory results.

Original languageEnglish (US)
Pages (from-to)316-335
Number of pages20
JournalApplied Psychological Measurement
Issue number4
StatePublished - Jun 2013


  • differential item functioning
  • ideal point model
  • item response theory
  • the generalized graded unfolding model (GGUM)

ASJC Scopus subject areas

  • Social Sciences (miscellaneous)
  • Psychology (miscellaneous)


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