Integrative Analytic Approach to Detecting and Interpreting L2 Vocabulary DIF

In this article we present results of a Differential Item Functioning (DIF) study using Shealy and Stout's (1993) Shealy, R. and Stout, W. F. 1993. A model-based standardization approach that separates true bias/DIF from group differences and detects test bias/DIF as well as item bias/DIF. Psychometrika, 58: 150–194. [Google Scholar] multidimensionality-based DIF analysis framework. In this framework, differences in test score distributions across different groups of examinees may be a result of multidimensionality if secondary dimensions (not the primary dimension measured by the test) differentially affect examinee performance. Thus, this framework requires both statistical and substantive judgments for identifying potential DIF items and substantiating the causes of DIF, which, as a result, will enhance a comprehensive construct validity argument. In this article, we illustrate step-by-step procedures of multidimensionality-based DIF analyses using LanguEdge reading comprehension test data. Qualitative data from think-aloud verbal protocols were used to generate DIF hypotheses about differential functioning of vocabulary items between two groups of Indo-European and non-Indo-European L2 learners. Statistical Simultaneous Item Bias Test (SIBTEST; Shealy & Stout, 1993 Shealy, R. and Stout, W. F. 1993. A model-based standardization approach that separates true bias/DIF from group differences and detects test bias/DIF as well as item bias/DIF. Psychometrika, 58: 150–194. [Google Scholar]) was used to test the DIF hypotheses. The DIF results supported the hypotheses by flagging four uniform DIF items and one crossing DIF item. Post-hoc analyses of the DIF-flagged items were performed by visual inspection of group differences using TestGraf (Ramsay, 2001 Ramsay, J. O. 2001. TestGraf: A program for the graphical analysis of multiple-choice test and questionnaire data [Computer software and manual], Montreal, , Canada: McGill University. [Google Scholar]) and revisiting the qualitative verbal data and analyses of cognate types. The results showed that DIF items with large effect sizes were associated with the following: (a) translation-equivalent cognates; (b) word meaning determined independent of context, and (c) less frequent, more difficult distracter words than those in stems. In light of empirical evidence, the article discusses implications for test development and validation processes.