Here is one way: y <- rnorm(50) x <- cut(rnorm(50, mean=y, sd=0.25),c(-3,-1.5,0,1.5,3))
a <- rbind( int=c(1,0,0,0), ab=c(-1,1,0,0), ac=c(-1,0,1,0), cd=c(0,0,-1,1) ) ai <- zapsmall(solve(a)) contrasts(x) <- ai[,-1] reg <- lm(y ~ x, na.action=na.omit) # check tmp <- tapply( y, x, mean ) (tmp2 <- c(tmp[1], tmp[2]-tmp[1], tmp[3]-tmp[1], tmp[4]-tmp[3])) all.equal( as.vector(tmp2), as.vector(coef(reg)) ) hope this helps, -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Ricardo Arias Brito > Sent: Wednesday, June 03, 2009 11:51 AM > To: r-help@r-project.org > Subject: [R] contrasts > > > Hi all, > I want t use a contrasts in adjusted regression (lm()) for a factor > with four levels, compared the coefficients first with second, first > with third, ..., third with fourth. > Someone can help with a contrast matrix to be used. I tried and not yet > I obtained. > > Say: > > y <- rnorm(50) > x <- cut(rnorm(50, mean=y, sd=0.25),c(-3,-1.5,0,1.5,3)) > reg <- lm(y ~ x, contrasts = list(x="contr.sum")) > > > > Thank you, > > Ricardo > > > > Veja quais são os assuntos do momento no Yahoo! +Buscados > http://br.maisbuscados.yahoo.com > > ______________________________________________ > 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. ______________________________________________ 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.
