This study used the Rasch Model in examining item difficulty and student ability parameters of NECO Biology examination in Nigeria. To achieve this purpose, two research questions were formulated to guide this study. Ex-post facto research design was adopted and a sample of 2,500 Senior Secondary III students drawn through multi-stage proportionate random sampling technique across Nigeria was used for the study. The NECO Biology objective questions from 2016 to 2018 were used as the instruments for data collection. The data were analyzed using item difficulty and ability parameter logits of the WinstepsRasch Measurement Model computer program. The findings of the study showed that item difficulty parameters were appropriate but were not arranged hierarchically from the least difficult to most difficult item and students’ ability parameters were appropriately estimated by the examination items. It was recommended among others that NECO should consider using the Rasch Model in developing her examination items so as to have valid and reliable items with appropriate item difficulty parameters and person ability parameters that measure the intended unidimensional construct.
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