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 <[EMAIL PROTECTED]> 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" <[EMAIL PROTECTED]> > To: "S.O. Nyangoma" <[EMAIL PROTECTED]> > 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 <[EMAIL PROTECTED]> > >>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