title: The healthy worker effect: Do health problems predict participation rates in, and the results of, a follow-up survey?
abstract: OBJECTIVE
  To determine the impact of the healthy worker effect (HWE) as a bias for the external and internal validity of the follow-up assessment in prospective survey research. Specifically, the study examined (1) whether the health status of respondents at the baseline measurement influenced response at the follow-up survey (external validity) and (2) whether HWE is a threat to internal validity by differential attrition, i.e., whether associations between work and health at baseline differ between stayers and dropouts.
  
  
  METHODS
  In a two-wave questionnaire survey with a 2-year time lag comprising 6283 persons, 4392 responded at both time points (response rate 70%). Mental distress and somatic symptoms served as indicators of health. Role conflict and role clarity were indicators of work factors.
  
  
  RESULTS
  There were few differences in response rate at follow-up between persons with and without health complaints at the baseline measurement. As response rate increased incrementally with educational level, there seems to be a socio-educational bias, rather than a HWE bias on survey participation. Baseline relationships between work factors and health indicators were equal in magnitude among stayers and dropouts.
  
  
  CONCLUSIONS
  The health status of participants at baseline seems to have little impact on the external and internal validity of the follow-up assessment in prospective survey research. Hence, the findings provide little support to the HWE as a potential bias in prospective studies within occupational health research. A limitation of the study is that the findings do not inform about the impact of the HWE on participation in the baseline assessment.
authors.affiliation: National Institute of Occupational Health, Pb 8149 Dep, 0033, Oslo, Norway. morten.nielsen@stami.no.
authors.forename: Morten Birkeland
authors.initials: MB
authors.lastname: Nielsen
authors.affiliation: National Institute of Occupational Health, Pb 8149 Dep, 0033, Oslo, Norway. stein.knardahl@stami.no.
authors.forename: Stein
authors.initials: S
authors.lastname: Knardahl
mesh_terms: D015746:Abdominal Pain / Q000453:epidemiology; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001416:Back Pain / Q000453:epidemiology; D015982:Bias; D002637:Chest Pain / Q000453:epidemiology; D004522:Educational Status; D005260:Female; D005500:Follow-Up Studies; D006261:Headache / Q000453:epidemiology; D006306:Health Surveys / Q000706:statistics & numerical data*; D015989:Healthy Worker Effect; D006801:Humans; D008297:Male; D008875:Middle Aged; D019547:Neck Pain / Q000453:epidemiology; D009664:Norway; D016272:Occupational Health; D010146:Pain / Q000453:epidemiology*; D015995:Prevalence; D015203:Reproducibility of Results; D013315:Stress, Psychological / Q000453:epidemiology*; D055815:Young Adult
keywords: Attrition; Generalizability; Non-response bias; Participation; Research design