Re: [R] nls

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Tue 15 Aug 2006 - 23:31:23 EST

Prof Brian Ripley <ripley@stats.ox.ac.uk> writes:

> You problem is x^c for x = 0. If you intended only c > 1, try a starting
> value meeting that condition (but it seems that the optimal c is about
> 0.27 is you increase x slightly).

Surely you mean c > 0.

> nls(1/y ~ a+b*x^exp(c), start=list(a=1.16122,b=0.01565,c=0))
Nonlinear regression model
  model: 1/y ~ a + b * x^exp(c)
   data: parent.frame()

         a b c
 0.9944025 0.1953168 -1.1495206
 residual sum-of-squares: 0.03303464
> nls(1/y ~ a+b*x^c, start=list(a=1.16122,b=0.01565,c=exp(-1.1)))
Nonlinear regression model
  model: 1/y ~ a + b * x^c
   data: parent.frame()

        a b c
0.9944026 0.1953165 0.3167891
 residual sum-of-squares: 0.03303464

(but even setting c=exp(-1) triggers the error; there could be cause for robustifying the nls algorithm)  

> Why have you used ~~ ? (Maybe because despite being asked not to, you
> sent HTML mail?)
>
> On Tue, 15 Aug 2006, Xiaodong Jin wrote:
>
> > Is there anyway to change any y[i] value (i=2,...6) to make following NLS workable?
> >
> > x <- c(0,5,10,15,20,25,30)
> > y <- c(1.00000,0.82000,0.68000,0.64000,0.66667,0.68667,0.64000)
> > lm(1/y ~~ x)
> > nls(1/y ~~ a+b*x^c, start=list(a=1.16122,b=0.01565,c=1), trace=TRUE)
> >
> > #0.0920573 : 1.16122 0.01565 1.00000
> > #Error in numericDeriv(form[[3]], names(ind), env) :
> > # Missing value or an infinity produced when evaluating the model
> >
> >
> > ---------------------------------
> >
> >
> > [[alternative HTML version deleted]]
> >
> > ______________________________________________
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> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
>
> --
> Brian D. Ripley, ripley@stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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>

-- 
   O__  ---- Peter Dalgaard             ุster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)                  FAX: (+45) 35327907

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Received on Tue Aug 15 23:35:50 2006

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