R help - Mar 2005 - nl regression with 8 parameters, help!

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

Does this error always occur independently of the starting values that you 
provide? I guess so, because I think that the parameters in your equation are 
not identifiable, since the first term (a1 to a4) is identical to the second 
term (b1 to b4) with a1 = b1, -a2 = b2, a3 = -b3, and a4 = b4 .
Do you really want to have the same explanatory variable ("Tl") in
both terms?

Arne


On Wednesday 23 March 2005 16:28, Guillaume STORCHI
wrote:
> 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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-- 
Arne Henningsen
Department of Agricultural Economics
University of Kiel
Olshausenstr. 40
D-24098 Kiel (Germany)
Tel: +49-431-880 4445
Fax: +49-431-880 1397
ahenningsen at agric-econ.uni-kiel.de
http://www.uni-kiel.de/agrarpol/ahenningsen/