Measuring differential item and test functioning across academic disciplines

Differential item functioning (DIF) is when a test item favors or hinders a characteristic exhibited by group members of a test-taking population. DIF analyses are statistical procedures used to determine to what extent the content of an item affects the item endorsement of sub-groups of test-takers. If DIF is found for many items on the test, the final test scores do not represent the same measurement across groups in the population of test-takers. This is known as differential test functioning (DTF). DTF is of particular concern in tertiary level language tests, where test-takers often differ in academic discipline. This study examined the DIF and DTF of an in-house developed assessment designed to measure how well first year students of five academic disciplines achieved material over the course of a year of English language study. The DIF and DTF tests were performed using Rasch analysis, which controls for ability across groups, ensuring that items are only flagged if groups of test-takers of the same ability levels exhibit a significantly different probability of endorsing the item. The current analysis outlines the process for checking for DIF and DTF and finds that even though DTF is unlikely, there were several items that favored and hindered some majors. Recommendations for modification of items are made and the importance of establishing a process to check for DTF and DIF, especially when the test-takers are from different disciplines of study, is discussed.