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??rtler