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The Influence of Selective Participation in a Physical Activity Intervention on the Generalizability of Findings

Vehtari, Aki DSc (Tech); Reijonsaari, Karita MBA; Kahilakoski, Olli-Pekka MSc (Tech); Paananen, Markus MD, PhD; van Mechelen, Willem MD, PhD; Taimela, Simo MD, PhD

Journal of Occupational and Environmental Medicine: March 2014 - Volume 56 - Issue 3 - p 291–297
doi: 10.1097/JOM.0000000000000000
Original Articles

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.

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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.

Address correspondence to: Olli-Pekka Kahilakoski, MSc (Tech), Department of Industrial Engineering and Management, Aalto University, FI-00076 Aalto, Finland (

A.V. has received grant support from the Academy of Finland (grant 218248).

S.T. is the founder and chief executive officer in Evalua International Ltd, which was responsible for the health risk appraisal used. S.T. and W.v.M. are directors of Evalua Nederland BV.

A.V., K.R., O.P.K., and M.P. declare that they have no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Web site (

Copyright © 2014 by the American College of Occupational and Environmental Medicine