[R] lme: error message with random=~1

[Thomas Petzoldt]

Douglas Bates wrote:
> 
> I'm not sure what model you want to fit here.  To specify a random 
> effect in lme you need both a grouping factor and a model matrix.  The 
> error message indicates that lme is unable to determine a grouping 
> factor.  It would be correct syntax if you added a single level factor 
> to the data frame and used that but then the model fit would fail 
> because you would be trying to estimate a variance in a model where 
> there is no variation in the term.

O.k. I see and think I understand it.

> It seems to me that you are trying to estimate parameters in a 
> mixed-effects model without any random effects and lme can't do that.
> 

Yes, what I want is a model without any random effects to be tested 
against a model with random effects. I want to show, that the random 
effects are negligible but that we account for pseudo replicates and 
have tested this explicitely.

I'm not sure what is better: to leave the random effects in the model or 
  simply an LR test against a linear model fitted by lm. I've never seen 
such an example in the books. Or have I missed a global alternative here?

Thomas P.