R help - Apr 2003 - nls: Missing value or an Infinity produced when evaluating the model

Hi,
I am trying to fit a sigmoid curve to some data with nls but I am getting 
into some trouble.
Seems that the optimization method is getting down to some parameter 
estimates that make the equation unsolvable. This is an example:
>growth<-data.frame(Time=c(5,7,9,11,13,15,17,19,21,23,25,27),BodyMass=c(45,85,125,210,300,485,570,700,830,940,1030,1120))
>GrowthModel<-nls(BodyMass~(((1-(1-((BirthMass/MaxMass)^0.25))*exp(-a*Time/(4*MaxMass^0.25)))^4)*MaxMass),data=growth,start=c(BirthMass=3,MaxMass=2500,a=1.5),trace=TRUE)
56043.86 :     3.0 2500.0    1.5
Error in numericDeriv(form[[3]], names(ind), env) :
        Missing value or an Infinity produced when evaluating the model

Is there anyway I can restrict the parameter values used so it doesn't get 
to this no return point.
Any other alternatives are also very welcome!
Thanks in advance,
Angel





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Hi Angel,

I tried reparametrise your model, setting:

BirthMass^0.25=u
MaxMass^0.25=v

and giving the following formula in R:

GrowthModel<-nls(BodyMass~(((1-(1-u/v)*exp(-a*Time/(4*v)))^4)*v^4),data=grow
th,start=c(u=4,v=5,a=1.5),trace=TRUE)

And this works for me, but the u estimate is negative (not significantly
different from 0, though).

Christian

----- Original Message -----
From: "Angel -" <angel_lul at hotmail.com>
To: <r-help at stat.math.ethz.ch>
Sent: Wednesday, April 23, 2003 2:29 PM
Subject: [R] nls: Missing value or an Infinity produced when evaluating the
model

> Hi,
> I am trying to fit a sigmoid curve to some data with nls but I am getting
> into some trouble.
> Seems that the optimization method is getting down to some parameter
> estimates that make the equation unsolvable. This is an example:
>
...