
> It's not a bug - it's a feature.  ML estimates or REML estimates of
> variance components can be zero.  This simply indicates that the
> variability in the response associated with the factor, RN in your
> case, is not sufficient to warrant the additional complexity in the
> model.
>    

does it mean that the correlation between two random effects can be 1 or 
-1?

Thanks,

Gustavo

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