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The study investigated detecting differential item functioning using item response theory in West African Senior School Certificate English language test in south-south Nigeria. 2 research questions were formulated to guide the study. Using descriptive research survey design for the study, study population was 117845 Senior Secondary 3 students in Edo, Delta, Rivers and Bayelsa state. A sample of 1309 (604 males, 705 females) drawn through multi stage sampling technique was used for the study. Two valid instruments titled: Socio-economic status questionnaire (SSQ) and WASSCE/SSCE English language objective test (ELOT) were used to collect data for the study. The reliability indices of the instruments were estimated using the Cronbach Alpha method of internal consistency and Richard Kuderson 20 with coefficient values of .84 for the English Language objective test and .71 for the socio-economic status questionnaire respectively. Chi-square and Lord Wald test statistics statistical technique employed by Item Response Theory for Patient Reported Outcome (IRTPRO) was the technique used in data analysis which provided answers to the research questions at.05 level of significance. On analysis, the result revealed that 13 items functioned differently significant between the male and female group and significantly 23 items differentially functioned between High and low socio-economic status group. Thus, this shows 18% DIF based on gender and 32% based on socio-economic status indicating large DIF and items that are potentially biased. Based on the findings, recommendation were made and one among others was that Item Response theory should be used as DIF detection method by large scale public examination and test developers.
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