Differences between participants and non-participants in an RCT on physical activity and psychological interventions for older persons.

BACKGROUND AND AIMS 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. CONCLUSION Due to volunteer bias, results of intervention studies on successful aging may have limited generalizability.