Fitting exponential curve to data points

Andrew Clegg <andrew.clegg <at> gmail.com> writes:

... If I want to demonstrate that a non-linear curve fits
better than an exponential, what's the best measure for that? Given
that neither of nls() or optim() provide R-squared. 

To supplement Karl's comment, try Douglas Bates' (author of nls) comments on the
matter

http://www.ens.gu.edu.au/ROBERTK/R/HELP/00B/0399.HTML

Short summary:
    * ... "the lack of automatic ANOVA, R^2 and adj. R^2 from nls is a feature,
not a bug :-)"
    * My best advice regarding R^2 statistics with nonlinear models is, as Nancy
Reagan suggested, "Just say no."

Dieter