% model-artifact difference
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\put(90,0){\vector(-1,0){18}}
\put(58,10){\vector(0,1){18}}
\put(13,28){\vector(0,-1){18}}
\put(102,28){\vector(0,-1){18}}
\put(0,0){\begin{parbox}{25\unitlength}
        {\begin{center}
        Predicted \\
        Behavior
        \end{center}}
        \end{parbox}}
\put(45,0){\begin{parbox}{25\unitlength}
        {\begin{center}
        Behavior \\
        Differences
        \end{center}}
        \end{parbox}}
\put(90,0){\begin{parbox}{25\unitlength}
        {\begin{center}
        Observed \\
        Behavior
        \end{center}}
        \end{parbox}}
\put(0,35){\begin{parbox}{25\unitlength}
        {\begin{center}
        System \\
        Model
        \end{center}}
        \end{parbox}}
\put(45,35){\begin{parbox}{25\unitlength}
        {\begin{center}
        Diagnosis
        \end{center}}
        \end{parbox}}
\put(90,35){\begin{parbox}{25\unitlength}
        {\begin{center}
        Actual \\
        System
        \end{center}}
        \end{parbox}}
\end{picture}