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Int J Epidemiol. 2010 Apr;39(2):417-20. doi: 10.1093/ije/dyp334. Epub 2009 Nov 19.

Illustrating bias due to conditioning on a collider.

Author information

1
Department of Epidemiology, UNC Gillings School of Global Public Health, Chapel Hill, NC, USA. cole@unc.edu

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.

PMID:
19926667
PMCID:
PMC2846442
DOI:
10.1093/ije/dyp334
[Indexed for MEDLINE]
Free PMC Article

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