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.

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