model_diagram.tex

\documentclass[convert={density=300,size=1200,outext=.png},tikz]{standalone}
%\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows}
\tikzset{ shorten <>/.style={ shorten >=#1, shorten <=#1 } }
% all other packages and stuff you need for the picture

\begin{document}
\begin{tikzpicture}
		%\draw[thick,dotted] (0,-0.25) rectangle (7.5, 1.25);
		%\node at (1, 0) [label=\large Compartment transitions] {};
		\draw (0, -1.25) circle(0.5) node (S) {$ S $};
		\draw (2, 0) circle(0.5) node (En) {$ E_n $};
		\draw (2, -2.5) circle(0.5) node (Eo) {$ E_o $};
		\draw (4, 0) circle(0.5) node (In) {$ I_n $};
		\draw (4, -2.5) circle(0.5) node (Io) {$ I_o $};
		\draw (6, 1.25) circle(0.5) node (Hn) {$ H_n $};
		\draw (6, -3.75) circle(0.5) node (Ho) {$ H_o $};
		\draw (8, -1.25) circle(0.5) node (R) {$ R $};

		\draw [thick,->,shorten >=3mm,shorten <=3mm] (S) -- (En) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (S) -- (Eo) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (En) -- (In) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (Eo) -- (Io) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (In) -- (Hn) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (Io) -- (Ho) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (In) -- (R) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (Io) -- (R) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (Hn) -- (R) node[midway,above] {};
		\draw [thick,->,shorten >=3mm,shorten <=3mm] (Ho) -- (R) node[midway,above] {};
		\end{tikzpicture}

\end{document}