Measurement issues related to the evaluation and monitoring of major depression prevalence in Canada.

Monitoring major depression prevalence is important because of the substantial impact of this condition on population health. Local or regional surveys using cost-efficient methods (e.g. data collection by telephone interview) may provide useful epidemiological data, as may the inclusion of brief diagnostic modules for major depression in general health surveys. In Canada, the Composite International Diagnostic Interview Short Form for Major Depression (CIDI-SFMD) has been widely employed for both purposes. The recent Canadian Community Health Survey 1.2 (2002), which employed a more detailed diagnostic interview (the World Mental Health 2000 CIDI), provides a standard against which to evaluate the performance of the CIDI-SFMD. A tendency to at times overestimate prevalence appears to be a characteristic of the CIDI-SFMD, and it has produced a broad range of prevalence estimates, suggesting a greater vulnerability to study-specific or contextual factors. However, the pattern of association of major depression with potential demographic determinants is not consistent with the classical "dilution" effect expected to occur with non-differential misclassification bias.