The term “selection bias” encompasses various biases in epidemiology. We describe examples of selection bias in case-control studies (eg, inappropriate selection of controls) and cohort studies (eg, informative censoring). We argue that the causal structure underlying the bias in each example is essentially the same: conditioning on a common effect of 2 variables, one of which is either exposure or a cause of exposure and the other is either the outcome or a cause of the outcome. This structure is shared by other biases (eg, adjustment for variables affected by prior exposure). A structural classification of bias distinguishes between biases resulting from conditioning on common effects (“selection bias”) and those resulting from the existence of common causes of exposure and outcome (“confounding”). This classification also leads to a unified approach to adjust for selection bias.
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From the *Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts; and the †Slone Epidemiology Center, Boston University School of Public Health, Brookline, Massachusetts.
Submitted 21 March 2003; final version accepted 24 May 2004.
Miguel Hernán was supported by NIH grant K08-AI-49392 and James Robins by NIH grant R01-AI-32475.
Correspondence: Miguel Hernán, Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115. E-mail: firstname.lastname@example.org