Thanks for your help. I can weight the variances in the 2 groups using "weights", as in
lme(Y~1+time+sex+age, random=~1|indv, correlation=corAR1(form=~time|indv), weights=varIdent(form=~1|sex),method="ML") but what I would like is to have a different "phi" estimate for each gender, not just different variances. Will track down a copy of the Pinheiro and Bates book... On 6/27/07, Bert Gunter <[EMAIL PROTECTED]> wrote: > Please read ?lme carefully -- the info you seek is there. In particular, the > weights argument for changing variance weighting by covariates and the > correlation argument for specifying correlation structures. > > Pinheiro and Bates's MIXED EFFECT MODELS IN S... is the canonical reference > (which you should get if you want to use R as you said) that exposits the > ideas at greater length. > > > Bert Gunter > Genentech Nonclinical Statistics > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Gareth Hughes > Sent: Wednesday, June 27, 2007 7:50 AM > To: r-help@stat.math.ethz.ch > Subject: [R] lme correlation structures > > Hi all, > > I've been using SAS proc mixed to fit linear mixed models and would > like to be able to fit the same models in R. Two things in particular: > > 1) I have longitudinal data and wish to allow for different repeated > measures covariance parameter estimates for different groups (men and > women), each covariance matrix having the same structure. In proc > mixed this would be done by specifying group= in the REPEATED > statement. Is this simple to do in R? (I've tried form=~time|indv/sex > for example but this doesn't seem to do the job). > > 2) I've read that other correlation structures can be specified. Does > anyone have any examples of how toeplitz or (first-order) > ante-dependence structures can be specified? > > Many thanks, > > Gareth > > ______________________________________________ > R-help@stat.math.ethz.ch 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@stat.math.ethz.ch 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.