biased. In contrast, Figure 3F shows a situation where selection will bias the association of the phenotype with the outcome, but the association of the SNP with the outcome will be unbiased. Other, more complex, situations can also lead to selection bias—we have not attempted to outline every possible case here. Algorithms for deciding whether a given causal analysis is biased by selection have been described,16 and could be used to decide whether bias is likely in a given case.
