Proper assessment leads to adequate judgement of examinees performance in external examinations. The method employed by any public examining bodies such as National Examinations Council (NECO) might be adduced to one of the reasons examinees' performances in Mathematics were fluctuating. Extant literature showed that NECO only established item and person parameters of their dichotomous and polytomous items using Classical Test Theory (CTT) which is sample-dependent, without exploring efficient frameworks by Item Response Theory (IRT) considering its efficacy for accurate calibration and...