Hi Fergus,
I think you can probably model this using SPM5's flexible factorial
set-up, with a factor for twin-pair and a factor identifying each twin
within each pair (and a factor for concordance, and/or a factor for
diagnosis -- I'm not sure from your description whether you need one
or both) in a similar way to longitudinal studies where there is a
subject factor and then other factors including time. There were a few
posts to the list recently about within-subjects designs.
You might also want the non-sphericity option for dependence over
levels of your "within-pair" (like within-subjects for a longitudinal
study) factor(s). Though my (limited) understanding of this is that:
1. It will only make a difference if you have more than two levels of
any within-subject factor(s), i.e. if you're only interested in a
contrast of concordant vs discordant then it shouldn't matter.
2. It won't give quite the same results as modelling dependency in a
conventional stats package (like STATA, SPSS or SAS), since the
variance components are estimated with data pooled over all voxels
that pass a certain "main-effects" threshold, rather than separately
for every voxel (which would be very slow, and possibly unstable).
I hope that helps,
Ged.
P.S. Please direct any replies to the list, rather than just to me;
firstly because that'll let other more knowledgeable people correct my
nonsense (!) and secondly because I might not be checking the list
quite so (over-) frequently as I usually do, for the next month or so.
Fergus Kane wrote:
> Dear All,
>
> This question has been asked once before but went unanswered. I am
> analysing twin pairs discordant (one has one has not a diagnosis) and
> concordant (both have) for a psychiatric disorder - as well as control
> pairs. The sample is - mixed identical and non identical.
>
> The problem is: most statistical tests assume independence of observation,
> but twins are clearly not independent. Depending on the model, this results
> in a violation of such assumptions between and within groups. I'd like to
> know if there is any way of addressing this issue within SPM?
>
> Many Thanks
>
> Fergus
>
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