title: Is self-selection the main driver of positive interpretations of general health checks? The Inter99 randomized trial.
abstract: OBJECTIVE
  To investigate if the lower mortality among participants of a health check followed by lifestyle intervention of high risk persons is explained by self-selection.
  
  
  METHODS
  All persons residing in the study area (Copenhagen; Denmark) were randomized to intervention (n=11,629) or control group (n=47,987). Persons in the intervention group were invited for a health check and individual lifestyle counselling. At baseline, 52.5% participated. Differences between participants and control group in 10-year all-cause and disease specific mortality was assessed. In survival analyses we controlled for socio-demography and mental and physical health.
  
  
  RESULTS
  Mortality rates were highest among non-participants and lowest among participants in the intervention group, whereas mortality rates of controls were approximately the average of those of participants and non-participants. In adjusted analyses the difference in both all-cause mortality (HR 0.73, 95% CI 0.62-0.85) and disease specific mortality was diminished between participants and controls, and turning insignificant as regard cancer mortality. In total, 39% of the lower mortality among participants in the intervention group was explained by self-selection according to socio-demography and health.
  
  
  CONCLUSIONS
  A substantial part of the lower mortality among participants in a preventive health check can be explained by confounding. The remaining lower mortality is most probably due to residual confounding, as the lower mortality was seen both for lifestyle related and non-lifestyle related diseases.
authors.affiliation: Research Centre for Prevention and Health, Building 84/85, Glostrup Hospital, DK-2600 Glostrup, Denmark. Electronic address: anne.mette.bender@regionh.dk.
authors.forename: Anne Mette
authors.initials: AM
authors.lastname: Bender
authors.affiliation: Research Centre for Prevention and Health, Building 84/85, Glostrup Hospital, DK-2600 Glostrup, Denmark; Faculty of Health Science, University of Copenhagen, Denmark; Faculty of Medicine, University of Aalborg, Denmark.
authors.forename: Torben
authors.initials: T
authors.lastname: Jørgensen
authors.affiliation: Research Centre for Prevention and Health, Building 84/85, Glostrup Hospital, DK-2600 Glostrup, Denmark.
authors.forename: Charlotta
authors.initials: C
authors.lastname: Pisinger
mesh_terms: D000328:Adult; D015982:Bias; D003718:Denmark; D005260:Female; D015438:Health Behavior; D006293:Health Promotion; D006304:Health Status; D006801:Humans; D008019:Life Style; D008297:Male; D008603:Mental Health; D008875:Middle Aged; D009026:Mortality; D010342:Patient Acceptance of Health Care; D012959:Socioeconomic Factors
keywords: Life style intervention; Mortality; Participation rate; Population level; RCT; Selection bias