Illustrating bias due to conditioning on a collider.
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.
Cole
Stephen R
SR
Department of Epidemiology, UNC Gillings School of Global Public Health, Chapel Hill, NC, USA. cole@unc.edu
Platt
Robert W
RW
Schisterman
Enrique F
EF
Chu
Haitao
H
Westreich
Daniel
D
Richardson
David
D
Poole
Charles
C
D015340:Epidemiologic Research Design; D006801:Humans; D016017:Odds Ratio; D012044:Regression Analysis; D018570:Risk Assessment / Q000379:methods; D015983:Selection Bias