R help - Oct 2003 - retrieving coefficients from nls() when it fails to converge

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



LEGAL NOTICE\ Unless expressly stated otherwise, this messag...{{dropped}}

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
>
>
>
>LEGAL NOTICE\ Unless expressly stated otherwise, this messag...{{dropped}}
>
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