The effects of the violation of local independence assumption on the person measures under the Rasch model

Abstract

Local independence of test items is an assumption in all Item Response Theory (IRT) models. That is, the items in a test should not be related to each other. Sharing a common passage, which is prevalent in reading comprehension tests, cloze tests and C-Tests, can be a potential source of local item dependence (LID). It is argued in the literature that LID results in biased parameter estimation and affects the unidimensionality of the test. In this study the effects of the violation of the local independence assumption on the person measures in a C-Test are studied. A C-Test battery comprising four passages, each containing 25 blanks, was analysed twice. Firstly, each gap was treated as an independent item and Rasch’s (1960) dichotomous model was employed. In the second analysis, each passage was treated as a super item and Andrich’s (1978) rating scale model was used. For each person, two ability measures were estimated, one on the basis of the dichotomous analysis and one on the basis of the polytomous analysis. The differences between the two measures, after being brought onto the same scale, are compared and the implications are discussed.