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Dear Jorge, > I have calculated the volume of activation of different brain areas > using a fixed effects analysis (fMRI data set, SPM99b). How can I > obtain an estimate of the error of these volumes? The distributional approximations for spatial extent will be found in: Friston KJ Worsley KJ Frackowiak RSJ Mazziotta JC & Evans AC Assessing the significance of focal activations using their spatial extent Human Brain Mapping 1994;1:214-220 These are used as the basis of inference (i.e. p values) based on spatial extent. Comparing two clusters however is not so simple, if that is what you want the error variance for. I hope this helps - Karl %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%