# Re: [R] Question about curve fitting...

From: Berton Gunter <gunter.berton_at_gene.com>
Date: Thu 11 Aug 2005 - 01:01:24 EST

As you appeared to have received no reply ...

1. Use nls() on the original equation
2. Transforming first and using linear fitting is **NOT** the same. The error structures differ and therefore you get different results. The greatest effect is on inferences -- i.e confidence intervals for the parameters: the usual asymptotic intervals would be symmetric based on the original scale, asymmetric based on the transformed. For reasonably well-behaved data the point estimates shouldn't be too much different, however. In any case, the linearization is a good way to find starting values.
• Bert Gunter Genentech Non-Clinical Statistics South San Francisco, CA

"The business of the statistician is to catalyze the scientific learning process." - George E. P. Box

> -----Original Message-----
> From: r-help-bounces@stat.math.ethz.ch
> [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of S.O. Nyangoma
> Sent: Wednesday, August 10, 2005 3:22 AM
> To: Dan Bolser
> Cc: R mailing list
> Subject: Re: [R] Question about curve fitting...
>
> I see that
> log(y)=log(k1)+k2*log(x)
> use lm?
>
> ----- Original Message -----
> From: Dan Bolser <dmb@mrc-dunn.cam.ac.uk>
> Date: Wednesday, August 10, 2005 11:41 am
> Subject: [R] Question about curve fitting...
>
> >
> > Meta:
> > This question is somewhat long and has two parts, I would be very
> > happyfor someone just to nudge me in the right direction with the
> > manual /
> > tutorial, as I am somewhat lost...
> >
> >
> > 1) How do I fit a curve of the form "y = k1 * x^k2" ?
> >
> > I want to estimate values of k1 and k2 given the x/y data I have,
> > and I
> > can't work out how to get R to calculate and return their estimates.
> >
> >
> > 2) Given the value of k1 and k2 for population A, how can I test if
> > population B has significantly different values of k1 and k2?
> >
> > Sorry for the basic question. I think I just need to read
> up on a few
> > functions.
> >
> >
> > I have about 50 xy pairs in total if that makes a difference.
> >
> > Dan.
> >
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>
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R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu Aug 11 01:08:10 2005

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