 Causal relationships among the variables in \(\bV\) are represented by graphs. We will consider 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 3 shows a directed graph on variable set \(\bV = \{S, T, W, X, Y, Z\}\).