title: The influence of selective participation in a physical activity intervention on the generalizability of findings.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  Selective participation may reduce the potential benefit of interventions and limit generalizability of findings.
authors.affiliation: From the Departments of Biomedical Engineering and Computational Science (Dr Vehtari) and Industrial Engineering and Management (Ms Reijonsaari and Mr Kahilakoski), Aalto University, Espoo, Finland; Institute of Clinical Medicine, Department of Physical and Rehabilitation Medicine, University of Oulu (Dr Paananen), Oulu, Finland; Department of Public and Occupational Health and EMGO+ Institute (Dr van Mechelen), VU University Medical Center, Amsterdam, the Netherlands; and Evalua International (Dr Taimela), Espoo, Finland.
authors.forename: Aki
authors.initials: A
authors.lastname: Vehtari
authors.forename: Karita
authors.initials: K
authors.lastname: Reijonsaari
authors.forename: Olli-Pekka
authors.initials: OP
authors.lastname: Kahilakoski
authors.forename: Markus
authors.initials: M
authors.lastname: Paananen
authors.forename: Willem
authors.initials: W
authors.lastname: van Mechelen
authors.forename: Simo
authors.initials: S
authors.lastname: Taimela
mesh_terms: D000328:Adult; D000367:Age Factors; D001499:Bayes Theorem; D015992:Body Mass Index; D003863:Depression / Q000523:psychology; D005260:Female; D015438:Health Behavior; D006293:Health Promotion; D006305:Health Status Indicators; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D016272:Occupational Health; D057185:Sedentary Behavior; D018582:Sick Leave; D017132:Workplace