Register-based study among employees showed small nonparticipation bias in health surveys and check-ups.

OBJECTIVES: To examine nonparticipation to a questionnaire survey and occupational health check-ups by sociodemographic variables and health status, measured by medically confirmed sickness absence, and whether the associations between other study variables and participation were affected by health status. STUDY DESIGN AND
SETTING: Questionnaire surveys and health check-ups were conducted among the City of Helsinki employees. Sample information was derived from the employer's personnel register and analyzed by participation and giving consent to link the data to external administrative registers.
RESULTS: Participation to the questionnaire survey was more common among the older, higher occupational classes, those with higher income, permanent employment, and those with no absence due to medically confirmed sickness. Among women in particular, the differences were small. Consent giving followed generally similar patterns than survey response. Nonparticipation to health check-ups was related to low income and temporary employment contract. In both questionnaire survey and health check-ups, associations between other study variables and participation were not affected by health status.
CONCLUSIONS: Questionnaire surveys and health check-ups were broadly representative of the target population. Associations between other study variables and participation did not differ by health status. This suggests that even when the data are not fully representative associations between the study variables need not to be biased.