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Hi Lindsay, I think this design should do it: EV1: intercept EV2: GDS EV3: CAPS EV4: GDS*CAPS EV5: age EV6: ICV The relevant contrasts are then: C1: 0 0 0 1 0 0 C2: 0 0 0 -1 0 0 All the best, Anderson On 23 May 2016 at 21:22, Lindsay Knight <[log in to unmask]> wrote: > Dear FSL experts, > > We are currently working on running structural analyses through FIRST. Our > sample is within group (all PTSD patients) and our two continuous variables > of interest are measures of depressive severity (GDS) and PTSD > symptamatology (CAPS). We also have two nuisance variables: age and ICV. > Specifically, we would like to look at the interaction of the two > behavioral measures (i.e. multiple regression of GDS x CAPS) while > controlling for age and ICV, in one model. Our investigation then, is to > see how the interaction of these behavioral measures predicts subcortical > volume and/or shape. We are having trouble, however, figuring out how to > set up the design matrix, contrast codes and F-tests. Any insights would be > greatly appreciated! > > Thank you, > > Lindsay Knight >