‘We thus proposed that a pre-exposure covariate *C* be considered a confounder for the effect of *A* on *Y* if there exists a set of covariates *X* such that the effect of the exposure on the outcome is unconfounded conditional on (*X*, *C*) but for no proper subset of (*X*, *C*) is the effect of the exposure on the outcome unconfounded given the subset.’ (VanderWeele & Shpitser, 2013, p. 215)

*The Annals of Statistics*,

*41*(1), 196–220.