[R] beginner gls (nlme) question

[Simon Blomberg]

Hi all,

I am trying to get a handle on gls (package nlme). I have a toy problem: 3 fixed factors (A, B, C), two levels each, 5 replicates per treatment. The response variable is continuous, normal. I have a correlation matrix of the form:

> mat
     A    B C
A 1.00 0.75 0
B 0.75 1.00 0
C 0.00 0.00 1

which is common to all observations.

How do I construct the call to gls? I think I need to use correlation=corSymm(), but I do not understand the precise syntax. I have read the relevant parts of Pinheiro and Bates, but they only talk about cases where the corSymm correlation structure is modelled, rather than known. I have also searched the R archives, but no luck.

I think it should be of the form gls(response~A*B*C, data=dat, correlation=corSymm(...?))
but I don't understand the arguments to corSymm.

Thanks in advance,

Simon.

Simon Blomberg, PhD
Depression & Anxiety Consumer Research Unit
Centre for Mental Health Research
Australian National University
http://www.anu.edu.au/cmhr/
Simon.Blomberg at anu.edu.au  +61 (2) 6125 3379