Dear Doug,

Thanks for your reply.

I've done as you suggested (your point b/), i.e. I've fit another model 
that considers 'couleurs' within 'nom'
However, after running anova() (likelihood ratio test), I find that I 
should keep the initial model that contains the -1 correlation:

 > anova (jb.lmer1, jb.lmer2)
Data: jb
Models:
jb.lmer2: lRT ~ couleurs + (1 | nom) + (1 | nom:couleurs)
jb.lmer1: lRT ~ couleurs + (1 + couleurs | nom)
          Df   AIC   BIC  logLik  Chisq Chi Df Pr(>Chisq)
jb.lmer2  5 17260 17295 -8625.2
jb.lmer1  6 17251 17293 -8619.4 11.584      1  0.0006654 ***

So, the question is now:

a/ How can I justify to refuse the initial model that contains the -1 
correlation?

b/ And a parallel question is: what is wrong with the -1 correlation? Is 
it because it is exactly -1 ? Would it still be a problem if it were -0.99 ?

c/ Ultimately, how can I report what appears to me as an important 
result: namely the high correlation between the intercept and the 
'couleurs' effect for each subject?


Thanks,
Eric





>> a/ is it really a statistical (or numerical) problem to have a -1
>> correlation in the model that I should keep?
>>      
>
> Yes, it is.  The fitted model is has a singular variance-covariance
> matrix for the random effects and that is not good.  In fact, it is no
> longer a linear mixed model.
>
>    
>> b/ is it possible to remove the correlation between Intercept and
>> Couleurs, as I would do if Couleurs were not a categorical factor?
>>      
>
> I would fit another model of
>
> IRT ~ couleurs + (1|nom:couleurs) + (1|nom)
>
> and see how that works.  This model is, in some sense, intermediate to
> the models that you have fit above.
>
>    
>> Thanks in advance,
>>
>> Eric Castet
>>
>>
>>
>>
>> --
>>
>> Eric Castet
>>
>> Institut de Neurosciences Cognitives de la Méditerranée -- INCM CNRS
>>
>> 31 chemin Joseph Aiguier
>>
>> 13402 Marseille cedex 20 (France)
>>
>> tel : (+33)(0)4-91-16-43-34
>>
>> fax : (+33) (0)4-91-16-44-98
>>
>> UMR 6193 du CNRS
>>
>> Université Aix-Marseille II
>>
>> http://www.incm.cnrs-mrs.fr/equipedyva.php
>>
>> http://www.incm.cnrs-mrs.fr/pperso/ecastet.php
>>
>>
>>
>>
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>>
>>
>> _______________________________________________
>> R-sig-mixed-models@r-project.org  mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>>
>>
>>      
>
>    

-- 

Eric Castet

Institut de Neurosciences Cognitives de la Méditerranée -- INCM CNRS

31 chemin Joseph Aiguier

13402 Marseille cedex 20 (France)

tel : (+33)(0)4-91-16-43-34

fax : (+33) (0)4-91-16-44-98

UMR 6193 du CNRS

Université Aix-Marseille II

http://www.incm.cnrs-mrs.fr/equipedyva.php

http://www.incm.cnrs-mrs.fr/pperso/ecastet.php


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