[R-sig-ME] Interaction between random and fixed effects

[Vinicius Maia]

Hi all,

I have a subtle doubt in how to interpret the interaction between fixed (or
even random) and random effects in the following case.

I have a model: Y ~ Year + (1|Local)+(1|Genotype) + (Year:Local:Genotype)
Year is a fixed effect because it has only 4 levels.

I ran the model with the random and fixed effect interaction just to
explore, but I was not expecting that it would work because Locals are
completely nested within Years.

To my surprise, the model ran and the variance of Year:Local:Genotype are
quite big. How is it possible to have Local interacting with Year if they
are nested? I also tried: Y ~ Year + (1|Local)+(1|Genotype) + (Year:Local)
and the model rans too, without singular fit.

I am struggling to understand if random interactions (it also extends to
cases where the interactions are only between nested random effects) mean
that the variance between Locals changes with Years or if the effect of a
given Local changes with Years. If it is the former option I can understand
why the model ran and has a high variance for the interaction, but if it is
the later case (which I believe it is), how does the model estimate an
interaction for Local:Year if they are nested?

Thanks!

Best wishes,

Vinícius Maia

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