Correlation Index

$\displaystyle r_c$	$\textstyle \equiv$	$\displaystyle {s_{y\hat y}\over s_ys_{\hat y}}$
$\displaystyle {r_c}^2$	$\textstyle =$	$\displaystyle {{s_{\hat y}}^2\over {s_y}^2} = 1-{{\rm SSE}\over {s_y}^2}.$