title: Participation bias in longitudinal studies: experience from the Population Study of Women in Gothenburg, Sweden.
abstract: OBJECTIVE
  To describe a cohort study of women receiving a series of comprehensive health examinations over 32 years.
  
  
  METHODS
  Longitudinal population study based on a randomised sample of the female population from defined age cohorts.
  
  
  METHODS
  City of Göteborg, Sweden.
  
  
  METHODS
  Subjects were 38, 46, 50, 54 or 60 years old at the start of the study in 1968. Re-examinations were performed in 1974, 1982 and 1992. Non-participants in the most recent examination, initiated in 2000, were offered home visits.
  
  
  METHODS
  Participation, anthropometric and blood pressure changes.
  
  
  RESULTS
  At the end of the 32-year follow-up, 64% of the original participants were alive, and low participation among survivors was a problem. An acceptable participation rate (71% of those alive) was obtained after home visits were offered. Surviving non-participants already had elevated cardiovascular risk factors at onset of the study in 1968, along with lower educational level and lower socioeconomic status. Home visited subjects were similar to non-participants with regard to anthropometry and blood pressure, but did not differ from participants with regard to social indicators. Thirty-two-year longitudinal data demonstrate clear ageing effects for several important variables, which should, however, be considered in the context of documented differences with non-participants at the baseline examination.
  
  
  CONCLUSIONS
  Longitudinal studies in elderly populations provide important data on changes during the ageing process. However, participation rates decline for a number of reasons and generalisations should be made with care. Moreover, including home visits in the protocol can both increase participation and reduce participation bias in elderly cohorts.
authors.affiliation: Department of Primary Health Care, Sahlgrenska Academy at Göteborg University, Göteborg, Sweden. lauren.lissner@medfak.gu.se
authors.forename: Lauren
authors.initials: L
authors.lastname: Lissner
authors.forename: Ingmar
authors.initials: I
authors.lastname: Skoog
authors.forename: Kate
authors.initials: K
authors.lastname: Andersson
authors.forename: Nils
authors.initials: N
authors.lastname: Beckman
authors.forename: Valter
authors.initials: V
authors.lastname: Sundh
authors.forename: Magda
authors.initials: M
authors.lastname: Waern
authors.forename: Dimitri Edin
authors.initials: DE
authors.lastname: Zylberstein
authors.forename: Calle
authors.initials: C
authors.lastname: Bengtsson
authors.forename: Cecilia
authors.initials: C
authors.lastname: Björkelund
mesh_terms: D000328:Adult; D000886:Anthropometry; D001794:Blood Pressure; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008875:Middle Aged; D018579:Patient Selection; D011159:Population Surveillance; D015983:Selection Bias; D013548:Sweden / Q000453:epidemiology; D016387:Women's Health