[R] nls, restrict parameter values

[Angel -]

Hi,
I posted a question (bellow) a few weeks ago and had a reply (thanks 
Christian) that partly solves the problem, but I still would like to be able 
to restrict some of the independent variables in a nls model to be always 
 >0, (is there a way to do it)??
Thanks,
Angel



>From: "Christian Ritz" <ritz at dina.kvl.dk>
>To: "Angel -" <angel_lul at hotmail.com>
>CC: <r-help at stat.math.ethz.ch>
>Subject: Re: [R] nls: Missing value or an Infinity produced when evaluating 
>the model
>Date: Wed, 23 Apr 2003 15:22:37 +0200
>
>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:
> >
> >> 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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