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Dear Hyo Jong, > Thank you for your reply. > That's what I thought first because Global Normalization is helpful > between subjects. > However, the spm_RandFX.man page describes the following at page 2: > "Global normalization and grand mean scaling should have been handled at > the within-subject level, so there is no need for such things at the > between subject level. Similarly, the contrast image produced are > implicitly masked, so there is no need to redefine the analysis volume > in the second level analysis." > > It does not seem to be consistent with my thought. > Any clarification please? No, it's more or less the same, as I wrote, i.e. there is no need for a scaling on the second (between subject) level. On the first (within subject) level, you could do the scaling, but usually, it's better to skip the scaling here, as well. Finally, the grand mean is 'handeled', i.e removed at the first level in any case, since you are modelling the overall session effect in your design matrix. So, no scaling on the within and between subject level is usually a good way to go. Good luck, Karsten -- ------------------------------------------------------------- Karsten Specht, PhD Department of Biological and Medical Psychology University of Bergen Jonas Lies vei 91 5009 Bergen Norway Tel.: +47-555-86279 Fax: +47-555-89872 [log in to unmask] http://fmri.uib.no/