Hey Dan,

I recently learned (at MHB) to use the \limits command to force the limits of sums and products to display where I want. The code:

r = \sqrt{1 - \frac{ \sum\limits_{i = 1}^n ( y_i - yf_i )^2 }{ \sum\limits_{i = 1}^n (y_i - \bar{y} )^2 }}

produces: