From: Gabor Grothendieck <ggrothendieck_at_gmail.com>

Date: Thu 11 Aug 2005 - 10:37:35 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 10:47:22 2005

Date: Thu 11 Aug 2005 - 10:37:35 EST

Note that even if you decide that this distinction is applicable, you may still wish to run a linear model prior to nls to get the starting values.

On 8/10/05, Dimitris Rizopoulos <dimitris.rizopoulos@med.kuleuven.be> wrote:

> I think here it's important to consider how the errors term come into

*> the model. If "y = k1 * x^k2 * e" then in the log-scale you have a
**> linear model; however if you assume that "y = k1 * x^k2 + e", the you
**> want a nonlinear model (i.e., nls()). For instance,
**>
**> x <- runif(500, 1, 3)
**> y <- 1 * x^2 + rnorm(500)
**> m <- nls(y ~ exp(k1 + k2 * log(x)), start = c("k1" = 1, "k2" = 2))
**> c(exp(coef(m)[1]), coef(m)[2])
**>
**>
**> I hope it helps.
**>
**> Best,
**> Dimitris
**>
**> ----
**> Dimitris Rizopoulos
**> Ph.D. Student
**> Biostatistical Centre
**> School of Public Health
**> Catholic University of Leuven
**>
**> Address: Kapucijnenvoer 35, Leuven, Belgium
**> Tel: +32/16/336899
**> Fax: +32/16/337015
**> Web: http://www.med.kuleuven.be/biostat/
**> http://www.student.kuleuven.be/~m0390867/dimitris.htm
**>
**>
**> ----- Original Message -----
**> From: "Dan Bolser" <dmb@mrc-dunn.cam.ac.uk>
**> To: "S.O. Nyangoma" <S.O.Nyangoma@amc.uva.nl>
**> Cc: "R mailing list" <r-help@stat.math.ethz.ch>
**> Sent: Wednesday, August 10, 2005 4:53 PM
**> Subject: Re: [R] Question about curve fitting...
**>
**>
**> > On Wed, 10 Aug 2005, S.O. Nyangoma wrote:
**> >
**> >>I see that
**> >>log(y)=log(k1)+k2*log(x)
**> >>use lm?
**> >
**> > Thats a nice solution in this instance, but in general how do I get
**> > R to
**> > fit a particular function (formula) and return the parameters?
**> >
**> > Cheers,
**> > Dan.
**> >
**> >>
**> >>----- 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
**> > 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
**> 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

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 10:47:22 2005

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