Abstract

CONDE, Frederico Neves  and  LAROS, Jacob Arie. Unidimensionality and the invariance property of IRT ability estimates. Aval. psicol. [online]. 2007, vol.6, n.2, pp.205-215. ISSN 1677-0471.

Item Response Theory (IRT) assumes the property of invariance of parameters, which permits to estimate the proficiency of a person independently of the test form administered. The present study investigated if estimates of proficiency are really independent of the difficulty of a test. The answers of 18,806 pupils attending the 8th grade of basic education to one of 26 Mathematics test booklets were analyzed. Results showed a dependency between the difficulty of the test booklet and estimates of proficiency, which becomes less strong after exclusion of items with weak loadings on the unidimensional factor. This finding suggests that the more the assumption of unidimensionality is satisfied, the weaker the relation between the difficulty of a test and estimates of proficiency. It can be concluded that verification of the assumption of unidimensionality is of vital importance always when IRT is used, otherwise the very desirable invariance property of IRT might not apply.

Keywords : Psychometrics; Item Response Theory (IRT); Invariance of parameters; Unidimensionality; SAEB.

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