This study examined the item bias of mathematics examination items constructed by West African Examination Council (WAEC) and National Examination Council (NECO) in Nigeria on the basis of probability of success of examinees of comparable ability. The study employed descriptive research design of the survey type. A sample of 600 Senior Secondary School three (SSS III) students was randomly selected from 12 Senior Secondary Schools in Ekiti State, Nigeria using multistage sampling technique. Two Instruments which included 50 mathematics test items randomly selected from past WAEC questions and 50 mathematics test items randomly selected from past NECO questions were used to collect data for this study. Two hypotheses were formulated and tested at 0.05 probability level. Two- parameter Logistic Bilog MG software statistical analysis was used to generate Item Response Theory (IRT) item characteristic curves (ICCs) for each NECO and WAEC Examination items to examine whether the items exhibit item bias or not . The results show that 44 (88%) of NECO items exhibit item bias while 35 (70%) of the WAEC items exhibit item bias. Hence, it was recommended that examination bodies should commit itself to eliminating or reducing biased items in National Examinations.
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