Comparison bias and dilution effect in occupational cohort studies

Stefano Parodi, Valerio Gennaro, Marcello Ceppi, Pierluigi Cocco

Research output: Contribution to journalArticlepeer-review


Health effects of occupational exposures are frequently evaluated by comparing the mortality of a whole cohort of workers with that of the general population. This study design may be affected by two major biases: a dilution effect (DE), due to the inclusion of unexposed subjects in the study cohort, and a comparison bias (CB), due to the different distribution of risk factors in the reference population. A theoretical model of the joint effect of DE and CB is proposed. Their impact was evaluated in two actual cohorts, selecting specific causes of death based on a priori hypotheses of an association. A linear relationship between the risk estimates and the two biases was found after applying either direct or indirect standardization to adjust for confounding. In the two cohorts, higher risks in exposed workers emerged only after adjusting for DE and CB. Cohort studies without an internal referent group may provide unreliable results.

Original languageEnglish
Pages (from-to)143-152
Number of pages10
JournalInternational Journal of Occupational and Environmental Health
Issue number2
Publication statusPublished - Apr 2007


  • Bias
  • Cancer
  • Healthy-worker effect
  • Occupational exposure

ASJC Scopus subject areas

  • Public Health, Environmental and Occupational Health


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