The underlying structure of diagnostic systems of schizophrenia: A comprehensive polydiagnostic approach Victor Peralta * , Manuel J. Cuesta Psychiatric Unit, Virgen del Camino Hospital, Irunlarrea 4, 31008 Pamplona, Spain Received 9 March 2005; received in revised form 3 May 2005; accepted 4 May 2005 Available online 1 July 2005 Abstract The objective was to ascertain the underlying factor structure of alternative definitions of schizophrenia, and to examine the distribution of schizophrenia-related variables against the resulting factor solution. Twenty-three diagnostic schemes of schizophrenia were applied to 660 patients presenting with psychotic symptoms regardless of the specific diagnosis of psychotic disorder. Factor analysis of the 23 diagnostic schemes yielded three interpretable factors explaining 58% of the variance, the first factor (general schizophrenia factor) accounting for most of the variance (36%). On the basis of the general schizophrenia factor score, the sample was divided in quintile groups representing 5 levels of schizophrenia definition (absent, doubtful, very broad, broad and narrow) and the distribution of a number of schizophrenia-related variables was examined across the groups. This grouping procedure was used for examining the comparative validity of alternative levels of categorically defined schizophrenia and an ordinal (i.e. dimensional) definition. Overall, schizophrenia-related variables displayed a dose-response relationship with level of schizophrenia definition. Logistic regression analyses revealed that the dimensional definition explained more variance in the schizophrenia-related variables than the alternative levels for defining schizophrenia categorically. These results are consistent with a unitary and dimensional construct of schizophrenia with no clear bpoints of rarityQ at its boundaries, thus supporting the continuum hypothesis of the psychotic illness. D 2005 Elsevier B.V. All rights reserved. Keywords: Psychosis; Schizophrenia; Diagnosis; Classification; Diagnostic criteria 1. Introduction Diagnosing schizophrenia has been problematic since the earliest clinical descriptions of the disorder, and the existence of basic disagreements in the concept is reflective of the large number of compet- ing diagnostic systems that have been proposed over the last hundred years. The magnitude of the prob- lem is well illustrated by the finding that diagnostic systems may vary as many as sevenfold in their rates of diagnosing schizophrenia (Endicott et al., 1982). Furthermore, diagnostic systems of schizophrenia have been criticized on the basis of their unestab- lished construct validity and arbitrariness (Fenton et 0920-9964/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.schres.2005.05.003 * Corresponding author. Tel.: +34 848 422488; fax: +34 848 429924. E-mail address: victor.peralta.martin@cfnavarra.es (V. Peralta). Schizophrenia Research 79 (2005) 217 – 229 www.elsevier.com/locate/schres