Methodological issues in using a polydiagnostic approach to define psychotic illness.

Although a polydiagnostic approach to the definition of psychotic disorder provides many advantages to the researcher, there are also disadvantages. A comparison between several different sets of operational criteria using the OPCRIT computer program on a consecutive series of 397 psychotic subjects is described. The results show that although current diagnostic procedures are generally reliable, such approaches can still only supplement skilled clinical judgement, and there remain many pitfalls for the unwary.