An assessment of Stout's index of essential unidimensionality

Abstract

A simulation study was conducted to evaluate the dependability of Stout’s T index of unidimensionality as used in his DIMTEST procedure. DIMTEST was found to dependably provide indications of unidimensionality, to be reasonably robust, and to allow for a practical demarcation between one and many dimensions. The procedure was not affected by the method used to identify the initial subset of unidimensional items. It was, however, found to be sensitive to whether the multidimensional data arose from a compensatory model or a partially compensatory model. DIMTEST failed when the matrix of tetrachoric correlations was non-Gramian and hence is not appropriate in such cases. Index terms: DIMTEST, essential unidimensionality, factor analysis, item response models, Stout’s test of unidimensionality, tetrachoric correlations, unidimensionality.