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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