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

[Douglas Bates]

Thomas Petzoldt wrote:
> 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.

I would recommend the likelihood ratio test against a linear model fit 
by lm.  The p-value returned from this test will be conservative because 
  you are testing on the boundary of the parameter space.