 If \(\bV\) is the set of variables included in a causal model, one way to represent the causal relationships among the variables in \(\bV\) is by a graph. Although we will introduce and use graphs in  section 3,  they will play a more prominent role in  section 4.  We will discuss two types of graphs. The first is the directed acyclic graph (DAG). A directed graph \(\bG\) on variable set \(\bV\) is a set of ordered pairs of variables in \(\bV\). We represent this visually by drawing an arrow from X to Y just in case \(\langle X, Y\rangle\) is in \(\bG\).  Figure 1  shows a directed graph on variable set \(\bV = \{S, T, W, X, Y, Z\}\).