The exposure odds ratio in nested case-control studies with competing risks.

A nested case-control study, also known as an ambidirectional study, is a case-control study within a cohort study. Although distortion by competing risks is well-recognized in follow-up studies, the problem has not been as widely appreciated in nested case-control studies. This paper extends previous work concerning the bias associated with competing risks for nested case-control studies. Specifically, the distorting effect of competing risks is illustrated for three methods of control selection. Assuming the proportional hazards model, the authors derived formulas for the bias of the odds ratio when competing risks cannot be ignored. Examples illustrate the magnitude of bias that occurs when the exposure of interest is associated with competing causes of death or withdrawal.