[R] effect sizes in lme/ multi-level models

[Leo Gürtler]

Dear alltogether,

I am searching for a way to determine "effect size" in multi-level 
models by using lme().
Coming from Psychology, for ordinary OLS there are measures (for 
meta-analysis, etc.) like

CohensD <- (mean_EG - mean_CG) / SD_pooled

or

(p)eta^2 <- SS_effect / (SS_effect + SS_error)

I do not intend to lead a discussion of the usefulness of such measures 
as long as the standards of psychological journals (e.g. as defined by 
the APA) order them.
However, I wondered how to determine measures of effect size in lme. 
Pinheiro&Bates (2000) do not touch that topic.
I assume that as long as a grouping structure is present, the formular 
of CohensD (see above) has to be corrected to give respect to the 
grouping structure. Is there any equivalent measure like eta^2, 
partial-eta^2, etc.?

Can anybody help me with formulas, R code or some references?

Thank you very much,

thanks in advance,

leo gürtler

-- 

email: leog at anicca-vijja.de
www: http://www.anicca-vijja.de/