R help - Apr 2005 - custom loss function + nonlinear models

Hi all;

I'm trying to fit a reparameterization of the
assymptotic regression model as that shown in
Ratkowsky (1990) page 96. 

Y~y1+(((y2-y1)*(1-((y2-y3)/(y3-y1))^(2*(X-x1)/(x2-x1))))/(1-((y2-y3)/(y3-y1))^2))

where y1,y2,y3 are expected-values for X=x1, X=x2, and
X=average(x1,x2), respectively.

I tried first with Statistica v7 by LS and
Gauss-Newton algorithm without success (no
convergence: predictors are redundant....). Then I
tried with the option CUSTOM LOSS FUNCTION and several
algorithms like Quasi-Newton, Simplex, Hookes-Jeeves,
among others. In all these cases the model converged
to some values for the parameters in it.

My question is (after searching the help pages) : Is
there such a thing implemented in R or can it be
easily implemented? In other words, is it possible to
define which loss function to use and the algorithm to
find the parameters estimates? 

Thanks
Christian

Christian Mora wrote:
>Hi all;
>
>I'm trying to fit a reparameterization of the
>assymptotic regression model as that shown in
>Ratkowsky (1990) page 96. 
>
>Y~y1+(((y2-y1)*(1-((y2-y3)/(y3-y1))^(2*(X-x1)/(x2-x1))))/(1-((y2-y3)/(y3-y1))^2))
>
>where y1,y2,y3 are expected-values for X=x1, X=x2, and
>X=average(x1,x2), respectively.
>
>I tried first with Statistica v7 by LS and
>Gauss-Newton algorithm without success (no
>convergence: predictors are redundant....). Then I
>tried with the option CUSTOM LOSS FUNCTION and several
>algorithms like Quasi-Newton, Simplex, Hookes-Jeeves,
>among others. In all these cases the model converged
>to some values for the parameters in it.
>
>My question is (after searching the help pages) : Is
>there such a thing implemented in R or can it be
>easily implemented? In other words, is it possible to
>define which loss function to use and the algorithm to
>find the parameters estimates? 
>
>Thanks
>Christian
>
>______________________________________________
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>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>
>
>
>  
>
try directly with optim()

Kjetil

-- 

Kjetil Halvorsen.

Peace is the most effective weapon of mass construction.
               --  Mahdi Elmandjra




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