The study determined the dimensionality of WAEC and NECO Economics test items and assessed the difference in each of item discrimination, difficulty and the guessing parameter of the two tests as obtained using CTT and IRT. These were with the view of determining the comparability of the two examinations under different test theories. The research design adopted for the study was descriptive. The population for the study consisted of secondary school students in Osun State and a sample of 540 students. The instruments used for the study were adopted respectively from the 2017 Economics WAEC and NECO Senior School Certificate Examination titled Economics Achievement Test 1 (EAT 1) and Test 2 (EAT 2). The results showed that the difference in the discrimination indices of NECO and WAEC Economics test items when CTT was used is not significant (U=1.52, P > 0.05). However, there is a significant difference (U=3.029, P < 0.05) in the discrimination indices when IRT was used. The results also showed that while the difference in difficulty indices of NECO and WAEC Economics test items was not significant with the use of CTT (U=0.138, P > 0.05), the difference was significant when IRT was used (U=2.095, P > 0.05). The results further showed that difference in the guessing parameter of NECO and WAEC Economics test items is not significant (U=1.519, P > 0.05). The results concluded that while the two examinations were comparable under classical test theory, they are not comparable under item response theory.