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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 >