Dear David, You might also take a look at the effects package, which can compute "adjusted" means and a variety of other adjusted effects.
I hope this helps, John > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Frank > E Harrell Jr > Sent: Thursday, May 27, 2004 1:45 AM > To: David J. Netherway > Cc: [EMAIL PROTECTED] > Subject: Re: [R] Getting the same values of adjusted mean and > standarderrors as SAS > > On Thu, 27 May 2004 16:34:58 +0930 > "David J. Netherway" <[EMAIL PROTECTED]> wrote: > > > Hello, > > > > I am trying to get the same values for the adjusted means > and standard > > errors using R that are given in SAS for the following > data. The model > > is Measurement ~ Age + Gender + Group. I can get the > adusted means at > > the mean age by using predict. I do not know how to get the > > appropriate standard errors at the adjusted means for Gender using > > values from predict. So I attempted to get them directly from the > > residuals as follows. The data is at the end of the email. > While there > > is a match for the males there is a large difference for > the females > > indicating that what I am doing is wrong. > > > > # > > meanAge <- mean(dd$Age) > > meanAgeM <- mean(dd$Age[d$Gender=="M"]) meanAgeF <- > > mean(dd$Age[d$Gender=="F"]) > . . . . > > By using sex-specific means of age you are not getting > adjusted estimates > in the usual sense. > > I prefer to think of effects as differences in predicted values rather > than as complex SAS-like contrasts. The Design package's > contrast function > makes this easy (including SEs and confidence limits): > > library(Design) # also requires Hmisc > d <- datadist(dd); options(datadist='d') > f <- ols(y ~ age + sex + group, data=dd) > contrast(f, list(sex='M'), list(sex='F')) # usual adjusted > difference M > vs F > contrast(f, list(sex='M',age=mean(dd$age[dd$sex=='M']), > list(sex='F',age=mean(dd$age[dd$sex=='F')) # M vs F not > holding age constant > > You can also experiment with specifying age=tapply(age, sex, mean, > na.rm=TRUE) using some of the contrast.Design options. > --- > Frank E Harrell Jr Professor and Chair School of Medicine > Department of Biostatistics Vanderbilt > University > > ______________________________________________ > [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
