You may want: lm( y ~ x:z )
This is the same model you fitted, but prametrized differently. But please check that what you REALLY want is not lm( y ~ z + x:z ) This is the model with different intercepts as well. Bendix Carstensen ---------------------- Bendix Carstensen Senior Statistician Steno Diabetes Center Niels Steensens Vej 2 DK-2820 Gentofte Denmark tel: +45 44 43 87 38 mob: +45 30 75 87 38 fax: +45 44 43 07 06 [EMAIL PROTECTED] www.biostat.ku.dk/~bxc ---------------------- > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of David Orme > Sent: Monday, March 29, 2004 4:44 PM > To: [EMAIL PROTECTED] > Subject: [R] Confidence Intervals for slopes > > > Hi, > > I'm trying to get confidence intervals to slopes from a linear model > and I can't figure out how to get at them. As a cut 'n' paste example: > > ################# > # dummy dataset - regression data for 3 treatments, each > treatment with > different (normal) variance > x <- rep(1:10, length=30) > y <- 10 - (rep(c(0.2,0.5,0.8), each=10)*x)+c(rnorm(10, sd=0.1), > rnorm(10, sd=0.6),rnorm(10, sd=1.1)) > z <- gl(3,10) > plot(y~x, pch=unclass(z)) > > # model as three slopes with common intercept > options(contrasts=c("contr.treatment","contr.poly")) > model <- lm(y~x+x:z) > > # coefficient table in summary gives the intercept, first > slope and the > difference in slopes > summary(model) > > # confint gives the confidence interval for the intercept and first > slope, > # and the CIs for the _differences_ > confint(model) > ################# > > What I'd like to report are the actual CI's for the slopes for the > second and third treatments, in the same way that confint returns the > parameter estimates for the first treatment. Can anyone point > me in the > right direction? > > Thanks, > David > > ______________________________________________ > [EMAIL PROTECTED] mailing list > https://www.stat.math.ethz.ch/mailman/listinfo> /r-help > PLEASE > do read the posting guide! > http://www.R-project.org/posting-guide.html > ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
