Background Public health monitoring depends on valid health and disability
estimates in the population 65+ years. This is hampered by high non-
participation rates in this age group. There is limited insight into size and
direction of potential baseline selection bias. Methods We analyzed baseline
non-participation in a register-based random sample of 1481 inner-city
residents 65+ years, invited to a health examination survey according to
demographics available for the entire sample, self-report information as
available and reasons for non-participation. One year after recruitment, non-
responders were revisited to assess their reasons. Results Five groups defined
by participation status were differentiated: participants (N = 299), persons
who had died or moved (N = 173), those who declined participation, but
answered a short questionnaire (N = 384), those who declined participation and
the short questionnaire (N = 324), and non-responders (N = 301). The results
confirm substantial baseline selection bias with significant
underrepresentation of persons 85+ years, persons in residential care or from
disadvantaged neighborhoods, with lower education, foreign citizenship, or
lower health-related quality of life. Finally, reasons for non-participation
could be identified for 78 % of all non-participants, including 183 non-
responders. Conclusion A diversity in health problems and barriers to
participation exists among non-participants. Innovative study designs are
needed for public health monitoring in aging populations
