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**Figure 1:** Fourth Figure in Lecture 9. This residual plot appears to have a horn-shape opening to the left, but in fact that is due to a concentration of points near the left side.
$\begin{figure} \centering \setlength {\unitlength}{1 in} \begin{picture} (6,5... ...thicklines \end{picture} \setlength {\unitlength}{1 pt} \protect\end{figure}$