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Just to add to the questions-- 1. CG has indicated to use "replications over subjects" and "no" to repeated measures while saying "yes" to non-sphericity correction. Why is that? Thanks in advance ... -Hima "Moo K. Chung" <[log in to unmask]> wrote: Although I don't think I saw a paper using overall GM or WM as nuisance parameters, I think Christian's idea is necessary. I did a simulated study last week on how GM or WM density changes under different SNR and CNR. They do change a lot. The Jacobian determinant is less sensitive to varying SNR and CNR. It has alsmot no effect on the Jacobian determinant within a reasonable change of SNR and CNR. So an alternative to Christian's idea would be putting SNR and CNR as nuisance parameters. Moo. On 9/15/05, Christian Gaser <[log in to unmask]> wrote: >For modulated data it is (depending on your hypothesis) recommended to include overall GM or >GM+WM estimates as nuisance parameter in your model. Christian > Hi, all? > > I am doing a VBM study to compare a patients group and a control group > by SPM2. > I have used VBM scripts from Christian Gasers > ( http://dbm.neuro.uni-jena.de/vbm.html) and generated the optimized > segemented gray matter files. If i want to compare the GMV (gray matter > volume), i should use the mGwxxx files (modulated file), but if i want > to compare GMC (gray matter concentration/density), i should use Gwxxx > (non-modulated) files directly, right? > > And, i am using the PET model to do the statistics (Compare-populations: > 1scan/subject, AnCova). I used the following parameters: > two groups (24 controls vs. 25 patients), > two nuisance variables(age, gender), > no global normalisation, > no grand mean scaling, > absolute masking(threshold = 0.1), > omit global calculation, > yes for non-sphericity correction, > relications over subject(25), > yes for correlated repeated measures. > > Are these parameters right? What are the meaning of relications over > subject(25) or over group (2), and the meaning of correlated repeated > measures? > > Thanka a lot for any suggestions. > > best,?? > > > ---------------------------- > Hengyi Rao, Ph.D. > [log in to unmask] > Department of Neurology > University of Pennsylvania > 3 W Gates > 3400 Spruce Street > Philadelphia, PA, 19104 > ---------------------------- > -- Moo Chung http://www.stat.wisc.edu/~mchung Assistant Professor Department of Statistics University of Wisconsin-Madison __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com