 |
 |
Many thanks to Ged for the very helpful explanation below, and to Volkmar, and Steve Fromm who answered off-mailbase. Regards - Mike >-----Original Message----- >From: SPM (Statistical Parametric Mapping) >[mailto:[log in to unmask]] On Behalf Of Ged Ridgway >Sent: Saturday, February 10, 2007 12:21 PM >To: [log in to unmask] >Subject: Re: [SPM] No modeled block effect in SPM5? > >Hi Mike, > >Thanks for emailing the figures. The difference is just that SPM2 >models a mean/constant column in addition to two columns for groups, >whereas SPM5 just models the group columns. As Volkmar said, the DF >are the same as the constant ones column is just the sum of the two >individual group columns (the rank of both design matrices is 3). > >The extra constant column in SPM2 means that the individual group >columns are no longer estimable on their own. A contrast of [1 0 0]' >over the spm5 design is equivalent to a contrast of [1 0 1 0]' over >the spm2 one, while [1 0 0 0] for spm2 is invalid. > >For the difference of the two groups (which I'm guessing is what you >are interested in), the spm5 contrast would be e.g. for A>B [1 0 0]-[0 >1 0] = [1 -1 0]. While for SPM2: [1 0 1 0] - [0 1 1 0] = [1 -1 0 0]. >So in both versions of SPM, a zero-padded contrast [1 -1] will give >the same correct answer for A>B. > >I worked through a related example, but for the case where someone was >testing the covariate, which might either be helpful or confusing: > >http://www.cs.ucl.ac.uk/staff/gridgway/ancova/ > >but I hope this email is helpful anyway. > >Ged. > > >Mike Glabus wrote: >> Firstly, I confess to a little ignorance for the rationale >for modeling >> the block effect but assume this is equivalent to the "DC" >or offset in >> the GLM, i.e. the y intercept (?). >> >> With that in mind, I have been attempting to replicate an >SPM2 design in >> SPM5 for a two-group VBM analysis. >> >> In SPM2 I used the "compare populations 1-scan per subject" >with ANCOVA. >> In SPM5 I have tried using both "independent t-test" and >"full factorial" >> models, but in both cases, there is no modeled block effect. >However, the >> df in both SPM2 and SPM5 designs is the same! (see attached) >> >> Is there an explantation for i) the absence of a modeleed >block effect in >> SPM5; ii) the df being the same in SPM2 and SPM5 designs, where one >> (putatively) should have one df less (SPM2). >> >> Regards - MFG >> >