Selection effects in psychiatric epidemiology: a 14-year prospective study of the Hopkins Symptom Check List-25 as a predictor of mortality in the Norwegian general population.

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 population-based 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 > or = 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.