Hi Simon, Thanks for your reply. Your reply reminds me that book. I've read it long time ago, but haven't try the weights option in my projects yet:)
Is the heteroscedastic test always less powerful because we have to estimate the within group variance from the given data? Should we check whether each group has equal variance before using weights=varIdent()? If we should, what is the function for linear mixed model? Thanks, Shirley On 6/27/07, Simon Blomberg <[EMAIL PROTECTED]> wrote: > The default settings for lme do assume equal variances within groups. > You can change that by using the various varClasses. see ?varClasses. A > simple example would be to allow unequal variances across groups. So if > your call to lme was: > > lme(...,random=~1|group,...) > > then to allow each group to have its own variance, use: > > lme(...,random=~1|group, weights=varIdent(form=~1|group),...) > > You really really should read Pinheiro & Bates (2000). It's all there. > > HTH, > > Simon. > > , On Wed, 2007-06-27 at 21:55 -0400, shirley zhang wrote: > > Dear Douglas and R-help, > > > > Does lme assume normal distribution AND equal variance among groups > > like anova() does? If it does, is there any method like unequal > > variance T-test (Welch T) in lme when each group has unequal variance > > in my data? > > > > Thanks, > > Shirley > > > > ______________________________________________ > > [email protected] 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. > -- > Simon Blomberg, BSc (Hons), PhD, MAppStat. > Lecturer and Consultant Statistician > Faculty of Biological and Chemical Sciences > The University of Queensland > St. Lucia Queensland 4072 > Australia > > Room 320, Goddard Building (8) > T: +61 7 3365 2506 > email: S.Blomberg1_at_uq.edu.au > > The combination of some data and an aching desire for > an answer does not ensure that a reasonable answer can > be extracted from a given body of data. - John Tukey. > > ______________________________________________ [email protected] 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.
