The study used differential item functioning (DIF) analysis to determine item bias in a chemistry aptitude test (CAT). A total of 270 items were administered, in paper and pencil format for item analysis purposes. Items were analysed by means of classical item analysis, IRT item analysis and DIF analysis in particular. The sample size was 554 secondary school students from SS1 and SS3. The study focused on selected secondary schools in Yoruba speaking states of the western part of Nigeria, Hausa speaking states in the northern part of Nigeria and Igbo speaking states in the eastern Nigeria. The sample was divided into subgroups which was used to investigate the DIF for the level of Education, gender, language and culture. This was used to indentify items that indicated bias in terms of gender, culture, language or level of education. Items that exceeded a predetermined amount of DIF were discarded from that final item bank, irrespective of which subgroup was being advantaged or disadvantaged. The process and results of the DIF analysis were discussed.
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