Résumé :
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[BDSP. Notice produite par INIST-CNRS oR0x8qG8. Diffusion soumise à autorisation]. Collider-stratification bias arises from conditioning on a variable (collider) which opens a path from exposure to outcome. M bias occurs when the collider-stratification bias is transmitted through ancestors of exposure and outcome. Previous theoretical work, but not empirical data, has demonstrated that M bias is smaller than confounding bias. The authors simulated data for large cohort studies with binary exposure, an outcome, a collider, and 2 predictors of the collider. They created 178 scenarios by changing the frequencies of variables and/or the magnitudes of associations among the variables. They calculated the effect estimate, percentage bias, and mean squared error. M bias in these realistic scenarios ranged from - 2% to - 5%. When the authors increased one or both relative risks for the relation between the collider and unmeasured factors to>8, the negative bias was more substantial (>15%). The result was substantially biased (e.g.,>20%) if an unmeasured confounder that was also a collider was not adjusted to avoid M bias. In scenarios resembling those the authors examined, M bias had a small impact unless associations between the collider and unmeasured confounders were very large (relative risk>8). When a collider is itself an important confounder, controlling for confounding would take precedence over avoiding M bias.
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