R help - May 2008 - my first post to the list

Hello R-listers! My first post to the list is a very simple one for those
who use the software continuosly. I am trying to understand the fixed-x
resampling and random-x-resampling method proposed by Fox about
Bootstrapping. The doubt that I have is on the side of the model run in one
of the functions expressed for fixed-x resampling. What I don't understand
is: X=model.matrix, and the -1 under mod= rlm. Please see below:
#fixed x-resampling
fit <- fitted(mod.duncan.hub)
e <- residuals(mod.duncan.hub)
X <- model.matrix(mod.duncan.hub)
boot.huber.fixed <- function(data, indices, maxit=20){
y <- fit + e[indices]
mod <- rlm(y ~ X - 1, maxit=maxit)
coefficients(mod)
}
duncan.fix.boot <- boot(Duncan, boot.huber.fixed, 1999, maxit=100)
duncan.fix.boot

I just need a quick explanation about WHAT the functions mean or do in this
context.
Thanks

	[[alternative HTML version deleted]]

Hello R-listers! My first post to the list is a very simple one for
those who use the software continuosly. I am trying to understand the
fixed-x resampling and random-x-resampling method proposed by Fox
about Bootstrapping. The doubt that I have is on the side of the model
run in one of the functions expressed for fixed-x resampling. What I
don't understand is: X=model.matrix, and the -1 under mod= rlm. Please
see below:
#fixed x-resampling
fit <- fitted(mod.duncan.hub)
e <- residuals(mod.duncan.hub)
X <- model.matrix(mod.duncan.hub)
boot.huber.fixed <- function(data, indices, maxit=20){
y <- fit + e[indices]
mod <- rlm(y ~ X - 1, maxit=maxit)
coefficients(mod)
}
duncan.fix.boot <- boot(Duncan, boot.huber.fixed, 1999, maxit=100)
duncan.fix.boot

I just need a quick explanation about WHAT the functions mean or do in
this context.
Thanks

Dear Rodrigo,

The model.matrix() function extracts the model matrix from the object
mod.duncan.hub. To see the contents of the model matrix, just print X
(i.e., enter X at the R command prompt). When you do this, you'll see
that the first column of X is the constant regressor, containing 1's.
The -1 in the subsequent model formula suppresses the column of 1s that
would otherwise be (redundantly) included by default.

A suggestion for posting to the r-help list: Use an informative subject
(e.g., "bootstrapping a regression model"): I almost missed your post.

I hope this helps,
 John

On Fri, 2 May 2008 11:54:28 -0600
 "Rodrigo Brice?o" <rjavierbriceno at gmail.com>
wrote:
> Hello R-listers! My first post to the list is a very simple one for
> those
> who use the software continuosly. I am trying to understand the
> fixed-x
> resampling and random-x-resampling method proposed by Fox about
> Bootstrapping. The doubt that I have is on the side of the model run
> in one
> of the functions expressed for fixed-x resampling. What I don't
> understand
> is: X=model.matrix, and the -1 under mod= rlm. Please see below:
> #fixed x-resampling
> fit <- fitted(mod.duncan.hub)
> e <- residuals(mod.duncan.hub)
> X <- model.matrix(mod.duncan.hub)
> boot.huber.fixed <- function(data, indices, maxit=20){
> y <- fit + e[indices]
> mod <- rlm(y ~ X - 1, maxit=maxit)
> coefficients(mod)
> }
> duncan.fix.boot <- boot(Duncan, boot.huber.fixed, 1999, maxit=100)
> duncan.fix.boot
> 
> I just need a quick explanation about WHAT the functions mean or do
> in this
> context.
> Thanks
> 
> 	[[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.
--------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/