To evaluate factors that characterize employees who did not participate in a physical activity intervention in an occupational setting and assess how selective participation affects inferences from the data.
Employees were asked to complete a health risk appraisal. The respondents were invited to participate in a physical activity intervention. We compared predictors of sickness absence (register data) among all respondents and those who participated in the intervention, using Bayesian regression analysis.
Of 1116 employees, 817 (73%) responded, of whom 544 (67%) participated in the intervention. Participants had better health behaviors and fewer health problems than those who did not participate. The predictors of sickness absence in all respondents differed from those who participated in the intervention.
Selective participation may reduce the potential benefit of interventions and limit generalizability of findings.