Hi -
Unfortunately, as you only have two runs per subject, you will not be able
to estimate a between-session variance separately for each subject.
What we advise under these circumstances is to make the assumption that
all subjects in a given group have the same between-session variance.
Then you estimate a single between-session variance for each group, which
will effectively be added onto the variance associated with the
mean of the measurements, and be taken up to the third level.
To do this, you could eaither have 1 big design matrix with 2 groups in
it, or (easier) one design matrix for each group, each of which will look
like:
GP EV1 EV2 EV3 ....
1 1 0 0 ....
1 1 0 0 ....
1 0 1 0 ....
1 0 1 0 ....
1 0 0 1 ....
1 0 0 1 ....
. . . . ....
. . . . ....
con: 1 0 0 ....
0 1 0 ....
0 0 1 ....
If you take the two DM approach, then you can run the group without your
new subject in, before you collect data from the new subject.
Hope this is useful
T
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Tim Behrens
Centre for Functional MRI of the Brain
The John Radcliffe Hospital
Headley Way Oxford OX3 9DU
Oxford University
Work 01865 222782
Mobile 07980 884537
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On Tue, 7 Sep 2004, Dost Ongur wrote:
> Hello,
>
> We have a study with 34 subjects in 2 groups (patient and control). Each
> subject had 2 identical runs.
>
> We are waiting to scan a final patient and have 35 subjects for the
> ultimate analysis. In the meantime, we would like to go ahead and finish
> the first and second level analyses for all existing data sets.
>
> Is it OK to do the second level analysis (across runs for each subject) for
> the 34 existing subjects in one large batch and then run the final
> subject's data when they become available on its own? Would we have to
> repeat the second level analysis once we have all 35 scans?
>
> Thanks,
> Dost Ongur
>
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