Have you considered "optim"? That has produced something for me when I got nothing from "nls". You may wish to call "optim" a second time with the output from the first; you might actually get different answers. Please specify "hessian=TRUE", as follows:

fit0 <- optim(..., hessian=TRUE)

Then "eigen(fit$hessian, symmetric=TRUE)" may tell you why "nls" did not converge.

The standard advice in such cases is to reconsider what you really want and then fewer parameters and / or reparameterize; see e.g., Bates and Watts (1988) Nonlinear Regression Analysis and Its Applications (Wiley).

hope this helps. spencer graves

Potter, David M wrote:

Hi --

I am fitting four-parameter logistic regression models using nls(),
typically within the body of a loop.  Sometimes a fit fails to converge and
yet I would like to pull off the coefficients from the last iteration and
use them to compute backfitted (i.e., inverse prediction) values.

Note: To ignore the fit completely, but have the loop continue, I have
successfully used try(), but in this case the fits are sufficient for
getting backfitted values (even though the coefficients are poorly
estimated), so I need the actual coefficients.

R information:

platform i386-pc-mingw32
arch i386 os mingw32 system i386, mingw32 status major 1 minor 7.1 year 2003 month 06 day 16 language R


Thanks for your help.

David

__
David M. Potter
Groton Nonclinical Statistics
Pfizer Global Research and Development



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