LDA in TIKZ

lda.tex

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}

\node (theta) at (0,0) [circle, draw] {$\theta$};
\node (z) at (2,0) [circle, draw] {$z$};
\node (w) at (4,0) [circle, draw] {$w$};
\node (alpha) at (1,2) [circle, draw] {$\alpha$};
\node (psis) at (5,2) [circle, draw] {$\Psi$};
\node (b) at (3,2) [circle, draw] {$\beta$};

\draw[->] (alpha) -- (theta);
\draw[->] (b.east) -- (psis.west);

\draw[->] (theta.east) -- (z.west);
\draw[->] (z.east) -- (w.west);
\draw[->] (psis) -- (w);

\draw (-.7,-.7) rectangle (6.,1);
\draw (1,-.5) rectangle (5,.75);
\draw (4.2,1.4) rectangle (5.8,2.6);

\node at (5.5,-.5) {$N$};
\node at (4.8,-.3) {$M$};
\node at (5.5,1.7) {$K$};
\end{tikzpicture}
\end{document}