Representativeness in population-based studies: a detailed description of non-response in a Danish cohort study.

Background: Decreasing rates of participation in population-based studies increasingly challenge the interpretation of study results, in both analytic and descriptive epidemiology. Consequently, estimates of possible differences between participants and non-participants are increasingly important for the interpretation of study results and generalization to the background population. Methods: An age-specific, population-based cohort of 1,198 individuals was examined at age 40, 45, 51, and 60. Participants were compared with non-participants and when possible also with the background population using a wide range of detailed information on somatic and mental health collected at each examination, including data from a clinical examination, biochemical measurements, questionnaires, interviews, and public registers. Results: Participation rates were higher than 80% at examinations at age 40, 45, and 51, but decreased to 65% at age 60. At the baseline investigation at age 40, analyses indicated that participants were representative of the cohort as well as the background population. However, the mortality rate was higher among non-participants in the succeeding 20 years. Among living cohort members at the 60-year examination, non-participants had lower socioeconomic status, higher hospitalization rate, and a worse overall health profile than participants. Conclusions: The detailed data presented reinforce the contention that the health profile of nonparticipants is typically worse than that of participants. The results also indicate that while data from public registers give easily accessible information about non-participants, these crude proxy measures of health may not be enough to document representativeness.