Background. The usefulness of any diagnostic scheme is directly related to its ability to provide
clinically useful information on need for care. In this study, the clinical usefulness of dimensional
and categorical representations of psychotic psychopathology were compared.
Method. A total of 706 patients aged 16–65 years with chronic psychosis were recruited.
Psychopathology was measured with the Comprehensive Psychopathological Rating Scale (CPRS).
Lifetime RDC, DSM-III-R, and ICD-10 diagnoses and ratings of lifetime psychopathology were
made using OPCRIT. Other clinical measures included: (i) need for care; (ii) quality of life; (iii)
social disability; (iv) satisfaction with services; (v) abnormal movements; (vi) brief
neuropsychological screen; and (vii) over the last 2 years – illness course, symptom severity, employment,
medication use, self-harm, time in hospital and living independently.
Results. Principal component factor analysis of the 65 CPRS items on cross-sectional
psychopathology yielded four dimensions of positive, negative, depressive and manic symptoms.
Regression models comparing the relative contributions of dimensional and categorical
representations of psychopathology with clinical measures consistently indicated strong and significant
effects of psychopathological dimensions over and above any effect of their categorical counterparts,
whereas the reverse did not hold. The effect of psychopathological dimensions was mostly
cumulative: high ratings on more than one dimension increased the contribution to the clinical
measures in a dose-response fashion. Similar results were obtained with psychopathological
dimensions derived from lifetime psychopathology ratings using the OCCPI.
Conclusions. A dimensional approach towards classification of psychotic illness offers important