The use of either 'broad' or 'restrictive' definitions of schizophrenia may result in vastly different samples on which follow-up data are reported. Systems with an inbuilt illness duration criterion, such as DSM-III and DSM-IIIR which require at least 6 months of unremitting symptoms and a decline in functioning, are likely to overselect patients already developing a chronic course. The result would be a greater homogeneity of outcome at the cost of a compromised representativeness of the sample as regards the range of possible outcomes of schizophrenia. Diagnostic systems that emphasize the cross-sectional features of the disorder, such as ICD-10 (which requires 1 month's duration of clinically characteristic symptoms) avoid this limitation, possibly at the expense of including some cases of good prognosis that may be aetiologically or pathogenetically different from poor prognosis schizophrenia. However, until aetiology is elucidated, or a validating pathognomonic lesion is established, the decision as to what constitutes 'true' schizophrenia will remain arbitrary. With regard to prognostic studies, less restrictive systems have the important advantage that a broad spectrum of outcomes would be available at the endpoint of prospective observation, allowing for subgroups to be identified and their characteristics related to the initial manifestations of the disorder and various risk factors.

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