Differential Item Functioning (DIF) is a critical step in the development and validation of educational and psychological instruments used for academic and research purposes in Nigeria. DIF analysis is a means of statistically identifying unexpected differences in performance across matched groups of examinees. The study compared the power of two statistics in detecting DIF in dichotomous and ordinal test items. These were with a view to improving the quality of test items construction. The study employed the descriptive-comparative research design. The population consisted of all undergraduate students who registered for EDU423 (Tests and Measurement) at Sule Lamido University during 2018/2019 Harmattan Semester. The sample consisted of an intact class of 513 Final Year undergraduate students who registered for the course. Thus, the entire population was therefore used, and no sampling was carried out. A null hypothesis was postulated to guide the study. Two self-developed instruments were used to collect data in the study. They were Undergraduate Students Achievement Test (USAT), and Students’ Efficacy Scale (SES). Data collected were analysed using Simultaneous Item Bias Test (SIBTEST), and Logistic Discriminant Function Analysis (LDFA) statistical methods. The results showed that there was a significant difference in the proportions of test items that function differently in the dichotomous and ordinal tests when the different methods are used (SIBTEST = 950.8, p<0.05; LDFA = 377.5, p<0.05). The study concluded that the two methods complement each other in their ability to detect DIF in the two test formats. It was recommended that DIF testing must be conducted especially for very important tests like educational and psychological instruments used by various researchers in all Nigerian Universities.
Keywords: DIF,SIBTEST, LDFA, Dichotomous Test, and Ordinal Test.