Collider bias | Catalog of Bias
A distortion that modifies an association between an exposure and outcome, caused by attempts to control for a common effect of the exposure and outcome When an exposure and an outcome independently cause a third variable, that variable is termed a ‘collider’. Inappropriately controlling for a collider variable, by study design or statistical analysis, results in collider bias. Controlling for a collider can induce a distorted association between the exposure and outcome, when in fact none exists. This bias predominantly occurs in observational studies. Because collider bias can be induced by sampling, selection bias can sometimes be considered to be a form of collider bias. The diagram below contrasts bias through confounding and collider bias. A clear example of collider bias was provided by Sackett in his 1979 paper. He analysed data from 257 hospitalized individuals and detected an association between locomotor disease and respiratory disease (odds ratio 4.06). The association seem