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.
Bibliographical noteFunding Information:
Grants R03-AI-071763, R01-AA-01759 and P30-AI-50410 (to S.R.C.); Lineberger Cancer Center Core Grant CA16086 and P30-AI-50410 (to H.C.); Intramural Research Program of the Eunice Kennedy Shriver NICHD, NIH; and the American Chemical Council to (to E.S.); P30-ES-10126 (to C.P.).