From: Dimitris Rizopoulos <dimitris.rizopoulos_at_med.kuleuven.be>

Date: Thu 11 Aug 2005 - 01:09:00 EST

Dimitris Rizopoulos

Ph.D. Student

Biostatistical Centre

School of Public Health

Catholic University of Leuven

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 01:12:29 2005

Date: Thu 11 Aug 2005 - 01:09:00 EST

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 Received on Thu Aug 11 01:12:29 2005

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