[R] Degree of freedom for contrast t-tests in lme

[S Nakagawa]

Dear all

Somebody may have asked this before but I could not find any answers in the web
so let me ask a question on lme.

When I have a fixed factor of, say, three levels (A, B, C), in which each level
has different size (i.e. no. of observations; e.g. A>B>C). When I run an lme
model, I get the same degree of freedom for all the contrast t-tests (e.g. AvsB
or BvsC). I have tried this to several data sets but the same thing happened.
Whatever sample size I have in different levels (in a fixed factor), I get the
same degree of freedom for t-tests. 

Why is this? Is this how mixed-effects model work? Does this mean that if I have
unbalanced design, results from lme are likely to be wrong?

Thank you for your help

Shinichi

-- 
Shinichi Nakagawa
Department of Animal & Plant Sciences,
University of Sheffield,	
Sheffield S10 2TN, UK
Tel: +44-114-222-0064  
Fax: +44-114-222-0002