Re: [R] syntax for interactions in lme

From: <Lorenz.Gygax_at_fat.admin.ch>
Date: Thu 27 Oct 2005 - 15:24:08 EST


> Host (fixed)
> Sire (random)
> Dam nested within Sire (random)
> Host * Sire (random)
> Host * Dam within Sire (random)
>
> So without the interactions I have:
>
> hogmodel = lme(gain ~ host, random = ~1|sire/dam)
>
> If I understand correctly, that "sire/dam" term gives me both
> Sire and Dam within Sire as random factors. OK, so now I want
> to add the two interactions (listed above)...

Correct, for the interactions write:

random = ~ host | sire/dam

an interaction between a fixed and a random term can be interpreted as a variability in the fixed term for the different levels in the random term. Thus the 1 (which stands for the intercept) is exchanged with the fixed effect(s) with which interactions are of interest.

This is easy if all the hierarchical levels of a nested random effects go into an interaction it is a bit more complicated if not. Say you only want the interaction of host and dam but not sire:

random = list (~ 1 | sire, ~ host | dam)

Hope this helps (it can all be found in Pinheiro and Bates and on the help pages).

Lorenz



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