[R] Linear Mixed Models with nlme, more than one random effect

[Giulio Di Giovanni]

Hi, I' trying to learn how to use lme for Linear Mixed Models  and I have a 
problem when I have to include more than one random effect in my model. I 
know that this could be a stupid question to ask, but I'm not able to solve 
it by myself... One example: if my model is

response = operator + block + day
with operator and block as fixed effects and day as random effect, I use

res.lme <- lme(resp ~ oper + block , random=~1|day)

If I want to include also another random effect, as "experiment", what I 
should do ?
This effect doesn't have to be nested, at the and I would like to have the 
COV matrix using (if I'm not wrong) getVarCov function.

Thanks in advance for any help or suggestions, I'm a beginner on this 
field...

Davide

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