Prevalence and incidence studies of schizophrenic disorders: A systematic review of the literature

Objective: To present the results of a systematic review of the literature published between January 1, 1980, and December 31, 2000, that reports findings on the prevalence and incidence of schizophrenia and related disorders. Method: We conducted a literature search of schizophrenia-related epidemiological studies, using Medline and HealthSTAR databases and canvassing English-language publications. We used a set of predetermined inclusion-exclusion criteria to identify relevant studies. Eligible publications were restricted to age ranges of 18 years and over for prevalence studies and 15 years and over for incidence studies. Prevalence and incidence data were extracted and analyzed for heterogeneity. Results: A total of 18 prevalence and 8 incidence studies met eligibility criteria for the review. Heterogeneity analysis revealed significant differences across 1-year and lifetime prevalence and 1-year incidence of schizophrenia. The corresponding pooled rates were: 0.34 per 100, 0.55 per 100, and 11.1 per 100 000, respectively; the variation in rates between studies was generally between 2- and 51-fold. Conclusions: Although we restricted this review to studies using rigorous and relatively homogeneous methods, there remains significant heterogeneity of prevalence and incidence rates. This strengthens support for the hypothesis that there is real variation in the distribution of schizophrenia around the world. Health planners need to have local data on schizophrenia rates to improve the accuracy of their interventions, while clinicians and researchers need to continue to investigate the etiology of this variation.