From: Berton Gunter <gunter.berton_at_gene.com>

Date: Thu 11 Aug 2005 - 01:01:24 EST

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

Date: Thu 11 Aug 2005 - 01:01:24 EST

As you appeared to have received no reply ...

- Use nls() on the original equation
- 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.
**> >
**> > ______________________________________________
**> > 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
**>
**> ______________________________________________
**> R-help@stat.math.ethz.ch mailing list
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**> PLEASE do read the posting guide!
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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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