Selection bias and nonresponse to health promotion in older adults.

Some epidemiologic studies have compared the characteristics of individuals who participate, refuse, and are unreachable in population studies, but results have been inconsistent. The Rural Health Promotion Project attempted to recruit all Medicare Part B noninstitutionalized individuals age 65-79 years in a rural community for a trial of preventive health services. Of 962 potential subjects, 360 (37.4%) participated, 253 (26.3%) refused, 176 (18.3%) were ineligible, and 152 (15.8%) were never reached by phone or mail. Approximately 3 years later, we reinterviewed the participants, refusals, and as many of the unreachables as possible. The 3-year mortality was similar for both refusals and participants (approximately 9%) but was much higher for ineligibles (29.0%) and unreachables (23.7%). Participants were more likely to have disease history, to have behavioral risk factors for disease, and to use health screening services. Refusals were the healthiest and possibly chose not to participate because they did not have risk factors targeted by the program. The unreachables had the highest prevalence of disability and health care inpatient reimbursement and may have been ineligible for the demonstration had they volunteered. We conclude that failure to reach potential participants for health promotion services may be a warning of "high risk."