RANDOM VERSUS VOLUNTEER SELECTION FOR A COMMUNITY-BASED STUDY

Background. Selection methods vary greatly in ease and cost-effectiven ess. The effects of selection factors associated with subjects' recrui tment into studies can introduce bias and seriously limit the generali zability of results. Methods. For an epidemiologic study, we recruited an age-stratified random sample of 1,422 community-dwelling individua ls aged 65+ years from the voter registration lists in a rural area of southwestern Pennsylvania. The first 1,366 of these were accrued thro ugh intensive recruitment efforts; the last 56 of them responded to a single mailing. To increase sample size for future risk factor analyse s, we also recruited by direct advertisement a sample of 259 volunteer s from the same area. The three groups were compared on selected basel ine characteristics and subsequent mortality. Results. The two subgrou ps of the random sample were not significantly different on any of the variables we examined. Compared to the random sample, in cross-sectio nal analyses, volunteers were significantly more likely to be women, m ore educated, and less likely to have used several health and human se rvices. Volunteers also had higher cognitive test scores and Instrumen tal Activities of Daily Living (IADL) ability. Over 6-8 years (10,861 person-years) of follow-up, volunteers had significantly lower mortali ty rates than randomly selected subjects. Conclusions. Health-related studies with populations composed partly or entirely of volunteers sho uld take potential volunteer bias into account when analyzing and inte rpreting data.