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.


METHODS
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.


CONCLUSIONS
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.
Nygård
Jan F
JF
Helse Øst Health Services Research Unit, Akershus University Hospital, Oslo, Norway. j.f.nygard@online.no
Klungsøyr
Ole
O
Sandanger
Inger
I
Svensson
Elisabeth
E
D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002423:Cause of Death; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D001523:Mental Disorders / Q000453:epidemiology* / Q000401:mortality; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D010555:Personality Inventory / Q000706:statistics & numerical data*; D011237:Predictive Value of Tests; D016016:Proportional Hazards Models; D011446:Prospective Studies; D012307:Risk Factors; D015983:Selection Bias