 A set of variables \(\mathbf{V}\) is causally sufficient if there is no variable W omitted from \(\mathbf{V}\), such that if it were added to \(\mathbf{V}\), it would be a direct cause of two variables in \(\mathbf{V}\). In  Figure 9,  \(\{U, W, Y\}\) is causally sufficient (assuming the original DAG is), but \(\{U, W, Y, Z\}\) is not, since W and Z have a common cause, V, that is left out of this set.