[R-sig-ME] observations in multiple groups

[Ben Bolker]

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On 11-06-08 11:39 PM, Jeroen Ooms wrote:
> The standard assumption in multilevel modelling seems to be that every
> observation is in 1 group. However, I have an application in which every
> observation is in 2 (exchangable) groups. More precisely, I have k groups
> and k^2 observations, and there is one observations for every combination of
> groups. The groups are unordered, so I can't do e.g. 1 + (1| firstgroup) +
> (1|secondgroup). Is there a way to model this with lme4?
> 
> So the schoolbook example would be a hierarchical model with children within
> classes, however every child is in two classes at the same time.
> 

  Isn't this exactly a crossed random effect?  i.e., child k is in
classes i and j, so their expected value (in the absence of any
covariates etc.) is (grand mean) + eps_i + eps_j +eps_(error).  I'm
sorry if I'm missing something, but it seems like (1|firstgroup) +
(1|secondgroup) is precisely what you would want in this situation ... ?
 Why does it matter if the groups are ordered?

  Ben Bolker
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