 A useful sufficient condition for identifiability is the back-door criterion (Pearl 1993, 2000 [2009: 79]). First we need to introduce some graphical terminology. A path between \(X\) and \(Y\) be a set of connected edges between \(X\) and \(Y\) going in any direction. \(Y\) is a descendant of \(X\) if there is a path from \(X\) to \(Y\) in which all the arrows go in the same direction. When \(X\) and \(Y\) are connected via a single path including a common cause such as \(X \leftarrow Z \rightarrow Y\), \(X\) and \(Y\) will  typically[6]  be unconditionally probabilistically dependent, but will be independent conditional on \(Z\). For such a path, we say that \(Z\) blocks the path between \(X\) and \(Y\). In contrast, when \(X\) and \(Y\) are connected by a path including a common effect, such as \(X \rightarrow Z \leftarrow Y\), then the path will be blocked provided that one does not condition on \(Z\) or a descendant of \(Z\). This reflects the fact that independent causes of a common effect will typically be dependent conditional on a common effect. An effect of \(X\) on \(Y\) is identifiable if there are no unblocked “back-door paths” between \(X\) and \(Y\): all paths that pass through common causes are blocked, and all other paths excepting those by which the cause influences its effect are open.