Jeremy:

Don't be silly. The model is overdetermined -- a and b "tradeoff" with each
other. e.g. for any solution (a,b), (a/k^m,b+m) for any m is also a
solution, where k = const (assuming I have correctly interpreted the model,
of course).

-- Bert

On Wed, Dec 19, 2012 at 11:59 AM, Jeremy Miles <jeremy.miles@gmail.com>wrote:

> Could you provide the code that you're running, so we can see what
> you're trying to do?  Even better would be a repeatable example.
>
> Jeremy
>
> On 19 December 2012 09:42, Yann Labou <yann.labou@outlook.com> wrote:
> > Hey all,
> >
> > I'm trying to fit a non-linear model y ~ a * constant ^ b * x ^ c and
> > estimates the paramaters a, b and c.
> >
> > Using the nls function, I'm getting following error message:
> >
> > Error in nlsModel(formula, mf, start, wts) :
> >   singular gradient matrix at initial parameter estimates
> >
> > If I logarithmize the whole equation log(y) ~ log (a) + b *
> log(constant) +
> > c * log(x) and fit the equation with lm, I get only NAs estimates for the
> > second term on the right side:
> >
> > Coefficients: (1 not defined because of singularities)
> >
> > Do you have any hints on how to fit this equation or any alternatives to
> > nls?
> >
> > Thanks,
> > Yann
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

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