Incidence of mental disorders in Edmonton: estimates of rates and methodological issues

An incidence study was conducted in Edmonton, Alberta, Canada, in which 3956 community residents were interviewed using the Diagnostic Interview Schedule (DIS) and a sample of 1964 subjects were reinterviewed with the DIS an average of 2.8 years later. Incidence rates, estimated for a range of DSM-III disorders, were surprisingly large, raising questions about the reliability of DIS data. We examined major depression in detail to uncover possible sources of unreliability. There were 138 'incident' cases of major depression, giving an annual incidence rate for both sexes of 27.9 (per 1000). However, based on reinterview data, 106 (80%) of the incident cases reported an age of onset prior to the initial interview. These findings appear to be the result of a difference between the DIS definition of age of onset and the one used in our analysis of incidence. A syndrome called depressive spell is defined and used to demonstrate that the large number of incident cases is likely due, at least in part, to incomplete recall of lifetime depressive symptoms. Despite these potential sources of bias, the possibility remains that incidence rates and lifetime risks in Edmonton are larger than has generally been reported in the literature.