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Unidimensional Interpretations for Multidimensional Test Items



This article considers potential problems that can arise in estimating a unidimensional item response theory (IRT) model when some test items are multidimensional (i.e., show a complex factorial structure). More specifically, this study examines (1) the consequences of model misfit on IRT item parameter estimates due to unintended minor item-level multidimensionality, and (2) whether a Projection IRT model can provide a useful remedy. A real-data example is used to illustrate the problem and also is used as a base model for a simulation study. The results suggest that ignoring item-level multidimensionality might lead to inflated item discrimination parameter estimates when the proportion of multidimensional test items to unidimensional test items is as low as 1:5. The Projection IRT model appears to be a useful tool for updating unidimensional item parameter estimates of multidimensional test items for a purified unidimensional interpretation.