\documentclass[12pt]{standalone}

\usepackage{ucs}
\usepackage[utf8x]{inputenc}

\usepackage[T1]{fontenc}
\usepackage{textcomp}
\renewcommand{\rmdefault}{ugm}
\renewcommand{\sfdefault}{phv}
\usepackage[garamond]{mathdesign}
\usepackage{tikz}
\usetikzlibrary{positioning, shapes, arrows, shadows}

\begin{document}

\tikzset{
  observed/.style={circle, draw},
  partly observed/.style 2 args={draw, fill=#2, path picture={
      \fill[#1, sharp corners] (path picture bounding box.south west) -|
      (path picture bounding box.north east) -- cycle;},
     circle},
    unobserved/.style={draw, circle, fill=gray!40},
    residual/.style={draw, rectangle}
}

\tikzset{>=latex}
\begin{tikzpicture}
  \node[observed] (y) {$Y$};
  \node[unobserved, above=of y] (x) {$X$};
  \node[observed, left=of x] (w) {$W$};

  \node[observed,right=of x] (z) {$Z$};

  \draw[->] (z) to (y);
  \draw[->] (z) -- (x);
  \draw[->] (x) -- (y);
  \draw[->] (x) -- (w);

  \draw[->] (x) to (y);

  \draw[->] (y) -- (w);
  \draw[->] (y) -- (w);


\end{tikzpicture}
\end{document}