  The paper provides a simple test for deciding, from a given causal diagram,
whether two sets of variables have the same bias-reducing potential under
adjustment. The test requires that one of the following two conditions holds:
either (1) both sets are admissible (i.e., satisfy the back-door criterion) or
(2) the Markov boundaries surrounding the manipulated variable(s) are identical
in both sets. Applications to covariate selection and model testing are
discussed.
