\documentclass{article}
\usepackage{com.braju.graphicalmodels}
\usepackage{amsmath}
% \special{papersize=40mm,80mm}
% \setlength{\pdfpagewidth}{40mm}
% \setlength{\pdfpageheight}{80mm}
% \usepackage[vcentering,dvips]{geometry}
% \geometry{papersize={80mm,120mm},total={80mm,120mm},margin=0mm}



\catcode`\@=11%

\pagestyle{empty}

\begin{document}
\TeXtoEPS

\LARGE
\psset{unit=10mm,arrowscale=1.5,linewidth=1.5pt}

\begin{pspicture}(0,0)(3,6)%\showgrid
  \SpecialCoor

  \rput(2,1){\GM@node[observed=true]{y_i}}  \GM@label[angle=0]{y_i}{$y_i$}  
  \rput(2,3){\GM@node{z_i}}                 \GM@label[angle=0]{z_i}{$z_i$}
  \rput(2,5){\GM@node[observed=true]{x_i}}  \GM@label[angle=0]{x_i}{$\boldsymbol{x}_i$}  
  
  \psset{arrows=->,nodesep=3pt}
  \ncline{x_i}{z_i}
  \ncline{z_i}{y_i}
  \nccurve[angleA=-130,angleB=130]{x_i}{y_i}  
    
  
\end{pspicture}

\endTeXtoEPS
\end{document}
