Andrew Clegg: > Great, thanks. 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.
You really need to *very* careful when trying to interprete R² (which can be defined in many nonequivalent ways) in the nonlinear case. Recommended (and, dare I say, *required* reading): Anderson-Sprecher R. (1994). ‘Model comparisons and R²’. The American Statistician, volume 48, no. 2, pages 113–117. DOI: 10.2307/2684259 Kvålseth T.O. (1985). ‘Cautionary note about R²’. The American Statistician, volume 39, no. 4, pages 279–285. DOI: 10.2307/2683704 Scott A. and Wild C. (1991). ‘Transformations and R²’. The American Statistician, volume 45, no. 2, pages 127–129. ISSN 0003-1305. DOI: 10.2307/2684375 The Scott & Wild paper has an example that looks very similar to yours, and that may be instructive. FYI, in case you’re not used to DOIs: you can resolve the above DOIs to fulltext URLs using http://dx.doi.org/ -- Karl Ove Hufthammer ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
