Let us compare a diagnosis of schizophrenia with a diagnosis of coronary heart disease. Few would disagree that their aetiologies, symptoms, clinical needs, treatments and outcomes are rather different; therefore, the diagnostic contrast they provide in the classification of medical disorders can be considered extremely useful, that is, has clinical validity (Kendell, 1989). The clinician will use the diagnosis to prescribe a particular treatment, the service provider will organize services around the specific treatment needs associated with each group, and the researcher will test specific hypotheses regarding different biological and psychosocial risk factors and mechanisms in each disorder.
The current systems of classification of mental disorders are based on the assumption that it is similarly useful to discriminate between, for example, schizophrenia, schizoaffective disorder, affective psychosis, delusional disorder and other psychosis. However, psychotic disorders may show considerable overlap in their aetiologies, risk mechanisms, manifestations, treatment needs and outcomes. The greater the degree of overlap, the less useful it becomes to discriminate between the various disorders, and the less likely it becomes that the different disorders exist as such in nature (that is, are valid entities). It is, therefore, important to investigate systematically the degree of overlap by sampling across conventional diagnostic categories. However, as the great majority of research effort has its focus specifically on an isolated diagnostic category, data regarding the relative validity of common diagnostic labels remain wanting.