Fig. 3.

Directed acyclic graph illustrating selection bias in a Mendelian randomization analysis of cancer prognosis. In this example, estimating the causal effect of body mass index on colorectal cancer survival, the sample is restricted to colorectal cancer cases. Conditioning analyses on colorectal cancer incidence (i.e., case status, a collider in this scenario) could generate a spurious association between two causes of colorectal cancer incidence (i.e., body mass index and cigarette smoking). This then induces an association between body mass index and colorectal cancer survival (via cigarette smoking) even in the absence of a true causal relationship between these two traits in the target population