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Hi Will, Thank's for your fast reply! OK the SPM.xX.X of the two models are different, but so are the SPM.xGX.rg (i.e. in the ANCOVA the globals are modelled explicitly in SPM.xX.X while the spm.xGX.rg are all 1. In the ANOVA the SPM.xX.X does not explicitly contain the globals, but they are present in SPM.xGX.rg). What does this mean? How do I know which subset of voxels have been used for the estimates of the sphericity correction? Hoping for more help.... Cheers Deny ----Messaggio originale----- Da: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] Per conto di Will Penny Inviato: mercoledì 30 marzo 2005 16.42 A: [log in to unmask] Oggetto: Re: [SPM] ANOVA/ANCOVA models and VBM D. Menghini wrote: > 1. I assume that my design matrix in the ANCOVA and ANOVA are identical > (since the GM-volumes are modelled in both cases). Is this true? > No, I don't think so. Have a look at the design matrices SPM.xX.X. Because the designs are different, SPM will potentially use a different subset of voxels to estimate the nonsphericity (see eg. line 472 in spm_spm.m) Best, Will.