which gives an unbiased estimate for the causal effect of "X" on "Y". The same adjustment formula works when there are multiple confounders except, in this case, the choice of a set "Z" of variables that would guarantee unbiased estimates must be done with caution. The criterion for a proper choice of variables is called the Back-Door and requires that the chosen set "Z" "blocks" (or intercepts) every path from "X" to "Y" that ends with an arrow into X. Such sets are called "Back-Door admissible" and may include variables which are not common causes of "X" and "Y", but merely proxies thereof.