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Dear Zhiqiang Zhang, SPM performs a voxel-wise statistical test. The number of degrees of freedom depends on your design, not on the volume covered by your mask. The shape and volume of your mask is important only to correct the significance levels for multiple comparisons. Therefore, one would normally not observe changes in T- or uncorrected p-values with and without a mask. Only corrected p-values and cluster-level statistics are likely to change, if the mask covers much less than whole brain volume. Volkmar Am Montag, den 14.07.2008, 10:49 +0800 schrieb Zhang Zhiqiang: > Dear SPMers, > > I want to compare the difference in a defined area between two groups > of subjects. A mask was first used to all data, so as to exclude the > data outside this defined area (was set to zero). this processing was > supposed to facilitate the detection of difference, because the "free > degree" will be decreased after mask. > > However, we found the result of 2-sample t-test (in the defined area) > didn't change a little (in contrast to that the mask was not used). I > think which might be owing that SPM is still taking account the "zero > value (outside the mask)" into t-test calculation. Therefore, I want > to know, whether SPM (SPM2) can perform t-test only within a mask? > > Thank you very much! > > Zhiqiang Zhang > > ______________________________________________________________________ > 雅虎邮箱，您的终生邮箱！ -- Volkmar Glauche - Department of Neurology [log in to unmask] Universitaetsklinikum Freiburg Phone 49(0)761-270-5331 Breisacher Str. 64 Fax 49(0)761-270-5416 79106 Freiburg http://fbi.uniklinik-freiburg.de/