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 Briceño" <[EMAIL PROTECTED]> 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@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/ ______________________________________________ R-help@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.