title: Differences between participants and non-participants in an RCT on physical activity and psychological interventions for older persons.
abstract: OBJECTIVE
  Volunteer bias in intervention studies on successful aging has been poorly explored. This paper investigated differences between participants and non-participants of the Groningen Intervention Study on Successful Aging (GISSA) over a wide range of demographic, physical, psychological and social subject characteristics.
  
  
  METHODS
  Subjects were recruited among a longitudinal cohort study (Groningen Longitudinal Aging Study) and included 558 men and 711 women, aged 65-96 years, who were invited to participate in the GISSA. Measures were obtained by questionnaires at the moment of invitation and eight years before invitation. Participants were compared with three groups of non-participants: persons who refused to participate, those who did not respond after a reminder, and those who intended to participate but withdrew before pre-test.
  
  
  RESULTS
  At the moment of invitation, participants were younger, better educated, and functionally and physically more active than the three groups of non-participants. They also had better scores on the physical functioning subscale of the medical outcome scale, better ADL, iADL and vigorous ADL functions and fewer depressive symptoms, and perceived less social support in everyday and problem situations. Participants reported a less strong rate of decline in physical and psychological functioning in the eight years prior to the invitation than did the other groups.
  
  
  CONCLUSIONS
  Due to volunteer bias, results of intervention studies on successful aging may have limited generalizability.
authors.affiliation: Centre for Human Movement Sciences, University of Groningen, Groningen, The Netherlands. M.J.G.van.Heuvelen@ppsw.rug.nl
authors.forename: Marieke J G
authors.initials: MJ
authors.lastname: van Heuvelen
authors.forename: Jacqueline B M
authors.initials: JB
authors.lastname: Hochstenbach
authors.forename: Wiebo H
authors.initials: WH
authors.lastname: Brouwer
authors.forename: Mathieu H G
authors.initials: MH
authors.lastname: de Greef
authors.forename: Gertud A R
authors.initials: GA
authors.lastname: Zijlstra
authors.forename: Ellen
authors.initials: E
authors.lastname: van Jaarsveld
authors.forename: Gertrudis I J M
authors.initials: GI
authors.lastname: Kempen
authors.forename: Eric
authors.initials: E
authors.lastname: van Sonderen
authors.forename: Johan
authors.initials: J
authors.lastname: Ormel
authors.forename: Theo
authors.initials: T
authors.lastname: Mulder
mesh_terms: D000203:Activities of Daily Living; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D015982:Bias; D004493:Education; D005260:Female; D005500:Follow-Up Studies; D015577:Geriatric Assessment; D006304:Health Status; D006801:Humans; D008297:Male; D001523:Mental Disorders / Q000523:psychology / Q000628:therapy; D008603:Mental Health; D009043:Motor Activity; D009426:Netherlands; D010358:Patient Participation / Q000523:psychology; D011613:Psychotherapy; D045808:Refusal to Participate / Q000523:psychology; D012944:Social Support; D011795:Surveys and Questionnaires; D035141:Voluntary Programs