Your model is almost certainly over-parameterized (given the data that you have to fit it), and the asymptotic correlation matrix of the parameters that you should get from the solutions that converged will probably have some large off diagonal elements. In other words, your model is essentially non-identifiable.
If you don't know what the above means, you shouldn't be using nls. Bert Gunter Genentech Nonclinical Biostatistics -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of Gabor Grothendieck Sent: Tuesday, March 30, 2010 4:25 AM To: Corrado Cc: r-help@r-project.org Subject: Re: [R] Error "singular gradient matrix at initial parameterestimates" in nls You could try method="brute-force" in the nls2 package to find starting values. On Tue, Mar 30, 2010 at 7:03 AM, Corrado <ct...@york.ac.uk> wrote: > I am using nls to fit a non linear function to some data. > > The non linear function is: > > y= 1- exp(-(k0+k1*p1+ .... + kn*pn)) > > I have chosen algorithm "port", with lower boundary is 0 for all of the ki > parameters, and I have tried many start values for the parameters ki > (including generating them at random). > > If I fit the non linear function to the same data using an external > algorithm, it fits perfectly and finds the parameters. > > As soon as I come to my R installation (2.10.1 on Kubuntu Linux 910 64 bit), > I keep getting the error: > > Error in nlsModel(formula, mf, start, wts, upper) : singular gradient > matrix at initial parameter estimates > > I have read all the previous postings and the documentation, but to no > avail: the error is there to stay. I am sure the problem is with nls, > because the external fitting algorithm perfectly fits it in less than a > second. Also, if my n is 4, then the nls works perfectly (but that excludes > all the k5 .... kn). > > Can anyone help me with suggestions? Thanks in advance. > > Alternatively, what do you suggest I should do? Shall I abandon nls in > favour of optim? > > Regards > > -- > Corrado Topi > PhD Researcher > Global Climate Change and Biodiversity > Area 18,Department of Biology > University of York, York, YO10 5YW, UK > Phone: + 44 (0) 1904 328645, E-mail: ct...@york.ac.uk > > ______________________________________________ > 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. ______________________________________________ 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.