[R] nls factor

[Manuel Morales]

Oops! I made a mistake. Corrected below.

On Wed, 2009-04-15 at 11:05 -0400, Manuel Morales wrote:
> A more compact way to code factors in nls is to use the syntax factor[].
> 
> Here's an example using a simplified version of Ravi's example:
> 
> n <- 200
> set.seed(123)
> x <- runif(n)
> a <- gl(n=2, k=n/2)  # a two-level factor
call the factor f instead of a:
f <- gl(n=2, k=n/2) # a two-level factor
> eps <- rnorm(n, sd=0.5)
> y <- as.numeric(a) * x^.5 + eps
change equation to reflect above change:
y <- as.numeric(f) * x^.5 + eps
> 
> nls(y ~ a[]*x^b, start=list(a=c(1,1), b=0.25))
most importantly, fix syntax:
nls(y ~ a[fac]*x^b, start=list(a=c(1,1), b=0.25))

Altogether:
n <- 200
set.seed(123)
x <- runif(n)
f <- gl(n=2, k=n/2) # a two-level factor 
eps <- rnorm(n, sd=0.5)
y <- as.numeric(f) * x^.5 + eps 

nls(y ~ a[fac]*x^b, start=list(a=c(1,1), b=0.25))

Sorry for any confusion!

Manuel

> Wed, 2009-04-15 at 10:22 -0400, Ravi Varadhan wrote:
> > Hi,
> > 
> > Here is one-way to do it (the following code shows a simulation example):
> > 
> > n <- 200
> > 
> > set.seed(123)
> > 
> > x <- runif(n)
> > 
> > f <- gl(n=2, k=n/2)  # a two-level factor
> > 
> > x1 <- x * (f == 1)
> > 
> > x2 <- x * (f == 2)
> > 
> > a <- c(rep(2, n/2), rep(5, n/2))
> > 
> > b <- 0.5
> > 
> > nsim <- 100
> > 
> > nls.coef <- matrix(NA, nsim, 3)
> > 
> > for (i in 1:nsim) {
> > 
> > eps <- rnorm(n, sd=0.5)
> > 
> > y <- a * x^b + eps
> > 
> > ans.nls <- try(nls(y ~ a1*x1^b + a2*x2^b, start=list(a1=1, a2=1, b=0.25)),
> > silent=TRUE)
> > 
> > if (class(ans.nls) != "try-error") nls.coef[i, ] <- coef(ans.nls)
> > 
> > }
> > 
> > apply(nls.coef, 2, summary)  
> >  
> > Hope this helps,
> > Ravi.
> > 
> > ----------------------------------------------------------------------------
> > -------
> > 
> > Ravi Varadhan, Ph.D.
> > 
> > Assistant Professor, The Center on Aging and Health
> > 
> > Division of Geriatric Medicine and Gerontology 
> > 
> > Johns Hopkins University
> > 
> > Ph: (410) 502-2619
> > 
> > Fax: (410) 614-9625
> > 
> > Email: rvaradhan at jhmi.edu
> > 
> > Webpage:  http://www.jhsph.edu/agingandhealth/People/Faculty/Varadhan.html
> > 
> >  
> > 
> > ----------------------------------------------------------------------------
> > --------
> > 
> > 
> > -----Original Message-----
> > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
> > Behalf Of Manuel Gutierrez
> > Sent: Wednesday, April 15, 2009 8:27 AM
> > To: r-help at r-project.org
> > Subject: [R] nls factor
> > 
> > I want to fit the model y=a*x^b using nls; where "a" should be different for
> > each level of a factor.
> > What is the easiest way to fit it? Can i do it with nls?
> > I've looked the help pages and the MASS example in page 249 but the formula
> > is different and I don't know how to specify it for my model.
> > Thanks,
> > Manuel
> > 
> > 	[[alternative HTML version deleted]]
> > 
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> > 
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
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