[R] nl regression with 8 parameters, help!

From: Guillaume STORCHI <nioniodesbois_at_yahoo.fr>
Date: Thu 24 Mar 2005 - 02:28:41 EST


I'm doing a non linear regression with 8 parameters to be fitted:

J.Tl.nls<-nls(Gw~(a1/(1+exp(-a2*Tl+a3))+a4)*(b1/(1+exp(b2*Tl-b3))+b4),data=Enveloppe,

                       start=list(a1=0.88957,a2=0.36298,a3=10.59241,a4=0.26308,
                                 

b1=0.391268,b2=1.041856,b3=0.391268,b4=0.03439))

   First, I fitted my curve on my data by guessing the parameters' values ("by hand"), and wrote them.

   Then, I ajusted my model only with two parameters (whereas the others were fixed with previously found values, I did it the same way for all parameters.

   Finally, I got 8 fitted values that I enventually embedded in my nls() function, like above, yet R talled me:
"Error in nlsModel(formula, mf, start) : singular gradient matrix at initial parameter estimates"

should I use optim() or optimize()?
How could I perform it?

Thanks for help

Guillaume Storchi



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