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 USâ€“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, 1â€“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.