title: Illustrating bias due to conditioning on a collider.
abstract: That conditioning on a common effect of exposure and outcome may cause selection, or collider-stratification, bias is not intuitive. We provide two hypothetical examples to convey concepts underlying bias due to conditioning on a collider. In the first example, fever is a common effect of influenza and consumption of a tainted egg-salad sandwich. In the second example, case-status is a common effect of a genotype and an environmental factor. In both examples, conditioning on the common effect imparts an association between two otherwise independent variables; we call this selection bias.
authors.affiliation: Department of Epidemiology, UNC Gillings School of Global Public Health, Chapel Hill, NC, USA. cole@unc.edu
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Cole
authors.forename: Robert W
authors.initials: RW
authors.lastname: Platt
authors.forename: Enrique F
authors.initials: EF
authors.lastname: Schisterman
authors.forename: Haitao
authors.initials: H
authors.lastname: Chu
authors.forename: Daniel
authors.initials: D
authors.lastname: Westreich
authors.forename: David
authors.initials: D
authors.lastname: Richardson
authors.forename: Charles
authors.initials: C
authors.lastname: Poole
mesh_terms: D015340:Epidemiologic Research Design; D006801:Humans; D016017:Odds Ratio; D012044:Regression Analysis; D018570:Risk Assessment / Q000379:methods; D015983:Selection Bias