The DIF-Free-Then-DIF strategy for the assessment of differential item functioning

Wen Chung WANG, Ching Lin SHIH, Guo Wei SUN

Research output: Contribution to journalArticle

24 Citations (Scopus)


The DIF-free-then-DIF (DFTD) strategy consists of two steps: (a) select a set of items that are the most likely to be DIF-free and (b) assess the other items for DIF (differential item functioning) using the designated items as anchors. The rank-based method together with the computer software IRTLRDIF can select a set of DIF-free polytomous items very accurately, but it loses accuracy when tests contain many DIF items. To resolve this problem, the authors developed a new method by adding a scale purification procedure to the rank-based method and conducted two simulation studies to evaluate its performances on DIF assessment. It was found that the new method outperformed the rank-based method in identifying DIF-free items, especially when the tests contained many DIF items. In addition, the new method, combined with the DFTD strategy, yielded a well-controlled Type I error rate and a high power rate of DIF detection. In contrast, conventional DIF assessment methods yielded an inflated Type I error rate and a deflated power rate when the tests contained many DIF items favoring the same group. In conclusion, the simulation results support the new method and the DFTD strategy in DIF assessment. Copyright © 2012 The Author(s).
Original languageEnglish
Pages (from-to)687-708
JournalEducational and Psychological Measurement
Issue number4
Publication statusPublished - Aug 2012



Wang, W. C., Shih, C. L., & Sun, G. W. (2012). The DIF-Free-Then-DIF strategy for the assessment of differential item functioning. Educational and Psychological Measurement, 72(4), 687–708.


  • Differential item functioning
  • Item response theory
  • Matching variables
  • Scale purification
  • Likelihood ratio tests