I take it the reason the \sum operator gives different "vertical" formats in the fractions than outside is to save space or something?

That is

$\displaystyle r = \sqrt{1 - \frac{ \sum_{i = 1}^n ( y_i - yf_i )^2 }{ \sum_{i = 1}^n (y_i - \bar{y} )^2 }}$

vs

$\displaystyle \sum_{i = 1}^n$

-Dan