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
>>
>
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