Jeremy: Don't be silly. The model is overdetermined -- a and b "tradeoff" with each other. e.g. for any solution (a,b), (a/k^m,b+m) for any m is also a solution, where k = const (assuming I have correctly interpreted the model, of course).
-- Bert On Wed, Dec 19, 2012 at 11:59 AM, Jeremy Miles <jeremy.mi...@gmail.com>wrote: > Could you provide the code that you're running, so we can see what > you're trying to do? Even better would be a repeatable example. > > Jeremy > > On 19 December 2012 09:42, Yann Labou <yann.la...@outlook.com> wrote: > > Hey all, > > > > I'm trying to fit a non-linear model y ~ a * constant ^ b * x ^ c and > > estimates the paramaters a, b and c. > > > > Using the nls function, I'm getting following error message: > > > > Error in nlsModel(formula, mf, start, wts) : > > singular gradient matrix at initial parameter estimates > > > > If I logarithmize the whole equation log(y) ~ log (a) + b * > log(constant) + > > c * log(x) and fit the equation with lm, I get only NAs estimates for the > > second term on the right side: > > > > Coefficients: (1 not defined because of singularities) > > > > Do you have any hints on how to fit this equation or any alternatives to > > nls? > > > > Thanks, > > Yann > > > > ______________________________________________ > > R-help@r-project.org 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. > > ______________________________________________ > R-help@r-project.org 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. > -- Bert Gunter Genentech Nonclinical Biostatistics Internal Contact Info: Phone: 467-7374 Website: http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org 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.
