[R] cross-classified random factors in lme without blocking

[Peter Dalgaard]

Douglas Bates <bates at stat.wisc.edu> writes:

> > (Sorry, I'm a little rusty on the syntax, but just follow the example
> > in P&B)
> > 
> > AFAIR, it also works with random=list(a=~1,one=~b) and vice versa.
> 
> Not sure about that.

Sorry. It's certainly not correct as written. It has to be something like

list(a=1,one=pdIdent(form=~b-1))
 
otherwise you get a general symmetric covariance for the effect of b.

> > (The model is the same but you get different DF calculations, none of
> > which are correct in the completely balanced case...)
> 
> I realize that it is awkward to use lme to fit models with crossed
> random effects.  As Saikat DebRoy and I described in a recent preprint
>         http://www.stat.wisc.edu/~bates/reports/MultiComp.pdf

.../MixedComp.pdf, right?

> we now have a good handle on the computational methods for
> mixed-effects models with nested or crossed or partially crossed
> random effects.
> 
> Both the nlme and the lme4 packages are based on structures that are
> tuned to nested random effects and do not easily accomodate crossed
> random effects.  I have a draft of the contents of classes and methods
> for fitting linear mixed-effects models with nested or crossed or
> ... but it is a long way from the draft to working, tested code.
> Although it will take some time to get all the pieces in place I do
> offer some encouragement that this awkward phrasing of crossed random
> effects will some day be behind us.

Looking forward to it... :-)

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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