The duration of major depressive episodes in the Canadian general population

The National Population Health Survey (NPHS) has provided a wealth of new data concerning major depression in the Canadian general population. The NPHS included a brief predictor of major depression, and also two questions (only one of which was asked of each subject) concerned with the duration of episodes in the preceding year. A striking finding was that many of the episodes identified were very brief. In this paper the NPHS data were examined from a different perspective in order to derive a complementary perspective on the episode duration data. Data from the 1994/95 and 1996/97 cycles of the NPHS were used in the analysis. The longitudinal data were used to generate approximations of age and gender-specific incidence for members of the population over the age of 12 years. An estimate of prevalence was made from the 1996/97 cross-sectional file. A basic expression relating prevalence to incidence and mean duration of illness was then applied within age and gender categories. Taken together, the incidence and prevalence data from the NPHS suggest a longer duration than was indicated by the NPHS interview duration item. A probable explanation is that the NPHS duration question had an upper limit of 52 weeks, whereas some episodes of major depression last longer than this. Particularly long episodes could have a large impact on mean duration in the population. Nevertheless, these data confirm the heterogenous nature of this condition; many people with the syndrome of major depression may have quite brief episodes.