Schizophrenia: Epidemiological Findings: Incidence and Prevalence

Epidemiological Findings: Incidence and Prevalence

The incidence of schizophrenia is defined as the number of new cases in a given population, usually per 1000 persons, during a specific period of time (1 year by convention). In an illness with an insidious onset, such as schizophrenia, accurate incidence rates can be difficult to determine. The incidence varies depend-ing on the methods and the diagnostic criteria used. For example, the US–UK study is often cited as an example of epidemiological variation based on different diagnostic criteria (Kramer, 1969). This study, conducted in the 1960s, found a lower incidence of schizophrenia in the UK than in the USA. It is now widely ac-cepted that this difference was found because a broader definition of schizophrenia was being used in the USA, and it did not re-flect true differences in the incidence of schizophrenia in each country.

The data obtained from World Health Organization (WHO) studies are important in part because the same diagnostic crite-ria were used in all countries studied. According to the results of the International Pilot Study of Schizophrenia, schizophrenia is found in all cultures and the incidence rates per 1000 people annually ranged from 0.15 in Denmark to 0.42 in India (WHO, 1973).

Because schizophrenia is a chronic illness, the incidence rates must, by definition, be much lower than the prevalence rates. Prevalence is defined as the number of cases present in a specified population at a given time or time interval (e.g., at a specific point in time, during a time period, or over a lifetime). Lifetime prevalence represents the proportion of persons who have ever had the illness at a given time. Lifetime prevalence rates of schizophrenia, based on the Epidemiology Catchment Area (ECA) data, were approximately 1% (range across three sites, 1–1.9%) (Robins et al., 1984). Point prevalence rates based on International Pilot Study of Schizophrenia data showed no significant differences across study centers: schizophrenia was found universally with relatively equal frequencies in a wide va-riety of cultures.