The application of taxometric analysis techniques to schizophrenia symptoms will likely yield only one answer, namely that all of the schizophrenia symptoms are dimensional, when studied in a sample that consists solely of psychotic patients. This is so because, on the assumption that schizophrenia represents a reasonably coherent class, then the analysis is being conducted completely "within" the class of schizophrenia affected (or psychotic) persons—there is no mixture at the latent level. Taxometric methods are intended to answer the "taxonic question," (namely, is the underlying nature of a construct dimensional or taxonic in nature?), in a large, relatively unselected sample of individuals. In short, it answers this question when it can begin with data that exist "across" or "between" classes—the method assumes that the data being analyzed can contain a mixture of latent classes.
Consider if one wanted to determine if mental retardation represented a latent taxon as indexed by intellectual functioning, yet one gathered IQ data only on subjects from an institution for the mentally retarded. One should not be at all surprised to find that retardation was "dimensional" due to the fact that one was looking at the data that come solely from "within" the retardation class. The same reasoning applies to the "dimensional" findings of Cuesta and colleagues. If one seeks to illuminate the latent structure of schizophrenia symptoms and uses a sample consisting only of psychotic patients, then one should not expect to find evidence of a latent discontinuity.
The taxonic question regarding the latent structure of schizophrenia symptom dimensions is best answered by application of the taxometric methods to large, epidemiologic databases where a large series of relatively unselected cases have all been rated for schizophrenia phenomenology.
Colleagues interested in the application of taxometrics to schizophrenia-related questions may find the following papers of interest (Lenzenweger and Korfine, 1992; Lenzenweger, 2003; 2004; Lenzenweger et al., 2007; Meehl, 1992; Waller et al., 2006).
Lenzenweger, MF. On thinking clearly about taxometrics, schizotypy, and genetic influences: Correction to Widiger (2001). Clinical Psychology: Science & Practice. 2003;10:367-369.
Meehl PE. Factors and taxa, traits and types, differences of degree and differences in kind. Journal of Personality. 1992;60:117-174.
Waller NG, Yonce LJ, Grove WM, Faust DA, and Lenzenweger MF. (2006). A Paul Meehl Reader: Essays on the Practice of Scientific Psychology. Mahwah, NJ: Lawrence Erlbaum and Associates, Inc.
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