 This version introduces the graphical notion of d-separation. A path from X to Y is a sequence of variables \((X = X_{1},\dots, X_{k} = Y)\) such that for each \(X_{i}\), \(X_{i+1},\) there is either an arrow from \(X_{i}\) to \(X_{i+1}\) or an arrow from \(X_{i+1}\) to \(X_{i}\) in \(\mathcal{G}\). For example, in  Figure 9,  \(\{U, Y, W, V, X\}\) forms a path. A variable \(X_{i}\), \(1 &lt; i &lt; k\) is a collider on the path just in case there is an arrow from \(X_{i-1}\) to \(X_{i}\) and from \(X_{i+1}\) to \(X_{i}\). In other words, \(X_{i}\) is a collider just in case the arrows converge on \(X_{i}\) in the path: 