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Dear SPMers, I have spent the last couple of hours searching the list for information on interactions with continuous variables in vbm (SPM2) but I am still very confused. I have two groups (A and B) and a covariate C. I would like to test the interaction between groups and the continuous variable C but I am not sure about the correct way to go about it. I have set up the following variables GroupA GroupB CovariateC 1 0 34 1 0 22 0 1 15 1 0 25 0 1 29 Etc.. I then was not sure whether to test the interaction between groups and covariates I should simply multiply groupA by covariateC to produce AxCInteraction and then use the following contrast GroupA GroupB CovariateC AxCInteraction 0 0 0 1 Or whether I should set up two interaction variables for each group variable and test the interaction with the following contrast GroupA GroupB CovariateC AxCInteraction BxCInteraction 0 0 0 1 -1 Your assistance would be greatly appreciated Nic