Hi,
I'm stuck with how one sets up their data frame to take into account relationships beyond that of twins (in my case - sibs and triplets).
For a standard twin model - your data is 'wide' formatted:
ie Var_t1 Var_t2
... ...
... ...
How do you do this for sibs - the closest I can think is to model a sib against each of their twin siblings, something like:
Var_t1 Var_Sib
Var_t2 Var_Sib for one family, then
Var_t1 Var_Sib
Var_t2 Var_Sib for another family, etc
and treat the relationship as you do for a DZ pair.
But then I have some twin pairs with multiple sibs - which would become considerably more complex. And also half a dozen triplets. I'm assuming I'm over-complicating things.
I'd appreciate any help.
Thanks,
Paul
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Essentially, the FIML methods assume that data on different rows are independent of each other. Therefore, it is necessary to organize possibly non-independent observations on one line (a wide format). Thus:
Var_t1 Var_t2 Var_Sib1 Var_Sib2
would be an example 'record' or row of the data frame. Note that this means that you need to specify a model with covariance structure capable of describing the largest pedigree, and all smaller ones (which would have missing data coded for non-existent relatives' data).
There is an alternative approach via multilevel modeling, but I don't know of any OpenMx implementation of this for unbalanced pedigrees.
Thanks Mike