Estimating incidence and prevalence of treated psychiatric disorders from routine statistics: the example of schizophrenia in Oxfordshire.

STUDY OBJECTIVE--To use routine statistical records to estimate the incidence and prevalence of treated schizophrenia. DESIGN AND SETTING--Analysis of linked records in Oxfordshire (population 540,000) for all people in contact with specialist psychiatric services from 1975-86. SUBJECTS--Records of 685 people with a diagnosis of schizophrenia as an inpatient and a further 294 people who received specialist psychiatric care for schizophrenia outside hospital without any record of inpatient care. MEASUREMENTS AND MAIN RESULTS--The measures most commonly recorded in psychiatric statistics, first admission rates for people in whom schizophrenia was recorded at their first psychiatric admission, were 8.7 per 100,000 males and 5.6 per 100,000 females. First contact rates for people in whom schizophrenia was recorded at any time in the study period and in any setting were 15.1 per 100,000 males and 11.4 per 100,000 females. Whichever patient population was analysed, the broad profile of schizophrenia by age, sex, and calendar time was similar. CONCLUSIONS--First admission rates for schizophrenia, as identifiable in current routine information systems, are useful indicators of the general pattern of disease but are inadequate absolute indicators of treated incidence. These data are limited to the first ever contact. Reliable information about the treated incidence of disease requires information systems which incorporate information about when and where each diagnosis was first made. Reliable information about treated prevalence requires systems which also incorporate data about death, recovery, and migration into and out of the study population.