A question I've been pondering recently is the derivation of the correlation coefficient for nonlinear regression. I'm more interested in the LA approach so I thought I'd ask here.
I just looked at this paper and noted the formula for r. (Second panel down on the left.) Is it really this simple a formula? If so what is the geometric meaning of it? I've never seen that form when talking about linear regression (just the form with all the sums and no geometric explanation.)
PS For those who don't want to look up the page
where yf_i are the "fitted" values.