\begin{tikzpicture}[->,>=stealth,shorten >=1pt,auto,scale=0.6,transform shape]
% x node set with absolute coordinates
\node[state] (U) at (0,0) {$U$};
\node[dstate] (X1) at (-1.5,-1.5) {$X_{1}^{(1)}$};
\node[dstate] (X2) at (-1.5,-3.5) {$X_{2}^{(1)}$};
\node[dstate] (X11) at (1.5,-1.5) {$X_{1}^{(2)}$};
\node[dstate] (X21) at (1.5,-3.5) {$X_{2}^{(2)}$};
\node[state] (C1) at (-1.5,-5.5) {$C_{1}$};
\node[state] (C2) at (1.5,-5.5) {$C_{2}$};
\node[state] (D) at (0,-7) {$D$};


% Directed edge
\path (U) edge (X1);
\path (X1) edge (X2);
\path (U) edge (X11);
\path (X11) edge (X21);
\path (X2) edge (C1);
\path (X21) edge (C2);
\path (C1) edge (D);
\path (C2) edge (D);

% Bidirected edge
%\path[bidirected] (A) edge[bend left=60] (C);
\end{tikzpicture}
