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Hi, I am doing aVBM group study using SPM. To account for brain size I am doing a global correction as follows 1. Calculate volumes for gray matter, white matter and CSF using spm_get_volumes. 2. Add these up and feed them as globals in the design specification. 3. Account for the globals by Proportional scaling or adding as a covariate. I notice that when I do this the t-scores increase quite a bit and I get blobs that survive whole brain correction (more of them survive if I do scaling instead of adding as a covariate). But if I don't include globals at all and don't perform any global normalization the t-scores are quite less and nothing survives whole brain correction. Is it normal to see this difference? Which is a better method of global normalization?