- - - Assistant Professor, Bahar Institute of Higher Education, Mashhad, Iran.
- - - Instructor, Bahar Institute of Higher Education, Mashhad, Iran.

The present mixed-method study aimed at investigating the presence of Differential Item Functioning (DIF) and Differential Skill Functioning (DSF) in a high-stakes language proficiency test in Iran, the English Proficiency Test (EPT) with different academic backgrounds (i.e., Humanities vs. Sciences & Engineering) using Item Response Theory (IRT) and Mantel-Haenszel (MH) approaches. It also aimed at detecting if there is any correlation between IRT and MH methods and also detecting which DIF items are biased. The English subtest consisted of a total of ۱۰۰ items. The participants (N = ۶۴۲) were selected by convenience sampling from universities in Tehran. The results displayed DIF between Sciences and Humanities students, but they did not show DSF in favor of a particular academic discipline group. Hence, on the basis of the findings, it was concluded that the EPT scores are not free of construct-irrelevant variance but because ۱۴ DIF detected items out of ۱۰۰ items were too small, the overall fairness of the test was confirmed. In addition, it was found that some of DIF detected items were bias and some of them functioned differently, simply because the two groups differed in their abilities. The positive correlation between IRT and MH methods was also proved.

differential item functioning, Differential Skill Functioning, Item Response Theory, Mantel-Haenszel, English Proficiency Test