BACKGROUND: Selection methods vary greatly in ease
and cost-effectiveness. The effects of selection factors
associated with subjects' recruitment into studies can
introduce bias and seriously limit the generalizability of
results.
METHODS: For an epidemiologic study, we
recruited an age-stratified random sample of 1,422
community-dwelling individuals aged 65+ years from the
voter registration lists in a rural area of southwestern
Pennsylvania. The first 1,366 of these were accrued
through intensive recruitment efforts; the last 56 of them
responded to a single mailing. To increase sample size for
future risk factor analyses, we also recruited by direct
advertisement a sample of 259 volunteers from the same
area. The three groups were compared on selected baseline
characteristics and subsequent mortality.
RESULTS:
The two subgroups of the random sample were not
significantly different on any of the variables we
examined. Compared to the random sample, in
cross-sectional analyses, volunteers were significantly
more likely to be women, more educated, and less likely to
have used several health and human services. Volunteers
also had higher cognitive test scores and Instrumental
Activities of Daily Living (IADL) ability. Over 6-8 years
(10,861 person-years) of follow-up, volunteers had
significantly lower mortality rates than randomly selected
subjects.
CONCLUSIONS: Health-related studies with
populations composed partly or entirely of volunteers
should take potential volunteer bias into account when
analyzing and interpreting data.