I'm wondering about whether Stata 9.0, or the upcoming
Stata 10.0 allows the user to fix elements of the
error variance matrix in the mixed model setting.
This is required for behavior genetic models described
by Guang Guo (2002: behavior genetics). Say, for
example, with xtmixed or a related command, is it
possible to do the following:
Take the multi-level model:
Yij = B0 + u_j + e_ij
where Yij is the observed linear outcome for
individual i in cluster j.
Can this model be specified so as to deal with
particular types of clusters, so that we could model:
Yij(t) = B0 + u_j(t) + e_ij(t)
where t indicates the type of genetic related ness
within the clusters of individuals, e.g. MZ twin, DZ
twin, full sibling, etc.
The variance of e_ij would be allowed to vary by
genetic relatedness (t) since "genetic theory expects
the within-cluster variance for more genetically
related clusters to be smaller."
Thanks,
Jacob Felson
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