R help - Jun 2007 - unequal variance assumption for lme (mixed effect model)

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

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
> 
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-- 
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

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