Investigating differential item functioning across interaction variables in listening comprehension assessment

Differential item functioning (DIF) analysis is essential to ensuring the equity of measurement for different subgroups at the item level and is an integral part of validity. However, existing DIF research often overlooks within-group heterogeneity, commonly assuming that test takers from different subgroups comprise a homogeneous population. This study investigated DIF across gender, academic background, and their interaction in listening comprehension assessment using Rasch measurement. It found that ignoring within-group heterogeneity would lead to the under-detection of DIF, likely due to the cancellation of DIF at broader group levels. In addition, the study is the first to investigate DIF in a linked test, a scenario more prevalent in practical testing. The findings of the study highlight the importance of accounting for within-group heterogeneity in test fairness investigations in language assessment research and point to the potential effect of test linking and equating on DIF analysis and interpretation.