Assessment of negative and positive symptoms in schizophrenia.

Reliable, convenient rating scales to assess negative and positive symptoms in schizophrenia are necessary to evaluate further the theoretical and clinical importance of this division of symptoms and signs. The authors describe the application of the Rasch model, a probabilistic, item-independent, and sample-independent test construction procedure to the development of scales for both types of symptoms. The scales for negative and positive symptoms, which are based separately on the Schedule for Affective Disorders and Schizophrenia-Current (SADS-C) and the Nurses' Observation Scale for Inpatient Evaluation (NOSIE), demonstrated excellent reliability and temporal stability (i.e., yielded a rank order of patients that remained stable over time). The pattern of interscale correlations supports the view that positive symptoms, cognitive-affective negative symptoms, and social withdrawal are independent of one another.