title: Comparison bias and dilution effect in occupational cohort studies.
abstract: 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.
authors: Epidemiology and Biostatistics Section, Scientific Directorate, G. Gaslini Children's Hospital, Genoa, Italy. stefanoparodi@ospedale-gaslini.ge.it
authors: Stefano
authors: S
authors: Parodi
authors: Valerio
authors: V
authors: Gennaro
authors: Marcello
authors: M
authors: Ceppi
authors: Pierluigi
authors: P
authors: Cocco
mesh_terms: D015982:Bias; D015331:Cohort Studies; D006801:Humans; D007854:Lead / Q000633:toxicity; D008669:Metallurgy; D008875:Middle Aged; D008962:Models, Theoretical; D009369:Neoplasms / Q000139:chemically induced / Q000401:mortality; D016273:Occupational Exposure / Q000009:adverse effects*; D011143:Polyvinyl Chloride / Q000633:toxicity; D012107:Research Design; D013997:Time Factors; D014752:Vinyl Chloride / Q000633:toxicity
chemical_list: D007854:Lead; D011143:Polyvinyl Chloride; D014752:Vinyl Chloride