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

Kjetil Brinchmann Halvorsen

2005-Apr-04 21:29 UTC

### [R] custom loss function + nonlinear models

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 > >______________________________________________ >R-help at stat.math.ethz.ch mailing list >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 -- Mahdi Elmandjra -- Internal Virus Database is out-of-date. Checked by AVG Anti-Virus.