Selection effects in psychiatric epidemiology

Background Inconsistent findings in studies examining the association between mental health problems and mortality have suggested study design as a source of discrepancy. This study investigates if selection bias is introduced by an extensive personal interview, recruiting a healthier population, and furthermore examines the association between mental distress and mortality. Method The OsLof study consists of a random populationbased sample of 2,014 persons above 18 years that participated in an interview in 1989, including the Hopkins Symptom Check List-25 (HSCL-25). A second material was drawn as an approximate 1% sample of the Norwegian population (N = 31.907). Both samples were followed up until 2003, emigration or death. Hazard rates were estimated by Cox regression and piecewise exponential survival regression. Results Mortality rates were similar regardless of HSCL, and lower than in the 1% population sample, in the first 2 years of follow-up. A higher mortality rate in those with HSCL C 1.75 appeared after 8 years of follow-up, and after 12 years the mortality rate was 1.57 times higher than in the population sample. Conclusion Studies of predictors ought to have a long follow-up, as several years might pass before differences in mortality are revealed. Due to selection effects, short follow- up might lead to erroneously believing predictors to be protective. This study was consistent with other studies with long follow-up, showing higher mortality associated with mental distress.