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Quantifying Biases in Causal Models: Classical Confounding Versus Collider-Stratification Bias

It has long been known that stratifying on variables affected by the study exposure can create selection bias. More recently it has been shown that stratifying on pre-exposure variables can induce confounding, even if no confounder is present to begin with. The present paper examines the relative magnitudes of these biases under some simple causal models, in which the stratification variable is graphically depicted as a collider (a variable directly affected by two or more other variables in the graph). The results suggest that bias from stratifying on variables affected by exposure and disease may ordinarily be expected to be comparable to bias from classical confounding (bias from failing to stratify on a common cause of exposure and disease), whereas other biases from collider stratification may tend to be much smaller.