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Confounding refers to asituation in which a measure of the effect of an intervention or exposure is distorted because of the association of exposure with other factor(s) that influence the outcome under investigation. This can lead to erroneous conclusions being drawn, particularly in observational studies.

Confounding by indication

This bias can arise in observational studies when patients with the worst prognosis are allocated preferentially to a particular treatment. These patients are likely to be systematically different from those not treated, or treated with something else.