THE EUROPEAN VERTEBRAL OSTEOPOROSIS STUDY (EVOS) - PARTICIPATION BEHAVIOR AND SELECTION BIAS IN GERMANY

Background: In population-based studies non-participation rates of abo ut one third of the sample can be expected. The number of refusals may even be higher, if personal attendence of the subjects is requested. A different participation behaviour of the diseased and non-diseased m ay affect the prevalence estimation of a disease as well as the risk f actor association. Methods and Results: The European Vertebral Osteopo rosis Study (EVOS) is an international, multicenter, cross-sectional s urvey in men and women aged 50 to 79 years. Within Europe 36 centres r ecruited 17 342 participants. The 8 German centres contribute about on e fourth of the data. In Germany subjects were recruited in several st eps: random sample drawing from population registries, initial postal questionnaire, medical interview, lateral X-rays of the thoracic and l umbar spine. Depending on the extent of participation a variable amoun t of sociodemographic data and information on subjective health is ava ilable from the nonreachable, non-responders, and responders with inco mplete and with complete examination. A comparison of the different pa rticipation groups showed, that especially old women were lost from th e study, Compared to those, who only answered to an initial questionna ire, subjects, who were interviewed and X-rayed, more often suffered f rom back pain but reported a better functional capacity. This could be a hint on a selection of a population of the ''worried well''. Conclu sion: On the whole the selection processes seem to have a minor influe nce on the outcome. The initial postal questionnaire turned out to be effective in collecting basic information from those who refused to at tend a personal examination.