title: Unaltered Image of Health Maintenance: An Observation of Non-Participants in a Swedish Cohort Study of 85 to 86 Year Olds.
abstract: BACKGROUND
  Selection bias is often inevitable in epidemiologic studies. It is not surprising that study conclusions based on participants' health status are frequently questioned.
  
  
  OBJECTIVE
  This study aimed to assess whether the non-participants affected the characteristics of a general population of the very old people.
  
  
  METHODS
  Prospective, cross-sectional (N=650, aged 85 years old) analysis and 1-year follow-up (n=273), in Linköping, Sweden.
  
  
  METHODS
  We analysed data on health-related factors from a postal questionnaire, a home visit and a clinic visit at baseline and at the 1-year follow-up. We calculated the effect size to evaluate the degree of differences between the groups.
  
  
  RESULTS
  A greater proportion of non-participants resided in sheltered accommodation or nursing homes (participants vs non-response vs refusal, 11% vs 22% vs 40, P<0.001, φ=0.24). During the home visit or clinic visit, a higher proportion of dropouts reported mid-severe problems in EQ-5D domains (mobility and self-care) and limitations in personal activities of daily living, but the differences between participants and dropouts were very small (φ<0.2). No significant difference was found between the groups with regard to emergency room visits or hospital admissions, despite the fact that more participants than dropouts (φ=0.23) had multimorbidities (≥2 chronic diseases). Living in sheltered accommodation or a nursing home (odds ratio (OR), 2.8; 95% confidence interval (CI), 1.5-5), female gender (OR, 1.8; 95% CI, 1.1-3.1) and receiving more home visits in primary care (OR, 1.03; 95% CI, 1-1.06) contributed positively to drop out in the data collection stages over the study period.
  
  
  CONCLUSIONS
  Non-participants were not considered to be a group with worse health. Mobility problems may influence very old people when considering further participation, which threatens attrition.
authors.affiliation: Huan-Ji Dong, Department of Geriatric Medicine, Linköping University, Linköping, Sweden, Tel.: +46 10 103 80 51; fax: +46 10 103 41 41. E-mail address: huanji.dong@liu.se.
authors.forename: H-J
authors.initials: HJ
authors.lastname: Dong
authors.forename: E
authors.initials: E
authors.lastname: Wressle
authors.forename: J
authors.initials: J
authors.lastname: Marcusson