Edith Cowan University, Western Australia in association with Khon Kaen University, Thailand and Bansomdejchaopraya Rajabhat University, Thailand.
The focus of this paper is to investigate the idea of differential ability for students ranking on a multiple choice test in the subject of physics at secondary level. The weakness of Classical Test Theory (CTT) in measuring the ability with continuity and consequently its ranking ability of students on the basis of ability has been highlighted in this paper. In CTT, a student attempting a difficult question and an easy question get equal credit which is not the case in Item Response Theory (IRT). Moreover in CTT two students with equal raw score have the same ranking while in IRT they have different ranking, making the job of policy maker easier to take decision. Two contemporary approaches, CTT and IRT were compared on their suitability for ranking and measuring true ability on teacher made test. The sample was 400, 9th grade students taken randomly from a variety of population in Pakistan. A content valid test of 80 multiple choice items was used as instrument .This attempt is an illustrative example that this problem can be overcome by using of Item Response theory in measurement. The implication of this work for the teachers is to give more stress on teaching cognitive skills rather than knowledge and for the policy maker to evaluate the students on the basis of their cognitive skills achievement to award scholarship and recruitment.