Abstract

Seventeen definitions of the severely and persistently mentally ill have appeared in the literature over the past decade. These definitions have been used by 13 authors to formulate service programs and to estimate the prevalence of serious mental illness in the population. To test the applicability of these definitions, the authors operationalized each definition and applied it to a representative sample of 222 patients receiving services in one of Philadelphia's inner-city neighborhoods. The analysis showed estimates of prevalence of serious mental illness ranging from 4% to 88% of the treated population, depending on the definition applied. The NIMH (1987) definition was representative of the middle-range