The multidimensionality of self-report schizotypy in a psychiatric population: an analysis using multidimensional Rasch models.

There is increasing empirical evidence from factor analytical studies that schizotypy is composed of three dimensions. All studies into the multidimensionality of schizotypy used common factor analysis of scales, either exploratory or confirmatory. We argue that for research into the multidimensionality of schizotypy with dichotomous item responses on questionnaires (as with the Schizotypal Personality Questionnaire [SPQ], Raine 1991) much can be learned using generalized multidimensional Rasch models (GMRMs). GMRMs require a priori postulated models of schizotypy, which can be tested in confirmatory analyses. We hypothesized four competing models of schizotypy, based on the literature and clinical impressions-two two-dimensional models and two three-dimensional models. We also hypothesized that items differ in the degree they are indicative of a particular dimension of schizotypy. The sample was 418 psychiatric inpatients and outpatients, with moderate levels of psychopathology, who filled in the SPQ. Both three-dimensional models yielded a much better fit to the data than both two-dimensional models. Our revised three-dimensional model, a revision of that by Raine et al. (1994) and Gruzelier (1996), yielded the best fit. It consisted of positive schizotypy, disorganization, and negative schizotypy. The results strongly suggest that schizotypy, as measured with the SPQ, is a three-dimensional construct.