Dear SPM-statisticians,
the "usual" way of calculating a fixed effects model for a group is to put the
individual time-series of each subject into one big first-level model (at
least to my knowledge). I have heard the computational burden to estimate
such a model is quite heavy (of course depending on the specific model).
That's basically why I have questions with regard to the following 3 points:
1) In case I already calculated the first level statistics for each subject
separately: Is there a way (formula) to calculate the fixed effects for that
group from their single subject's first level stats, without putting the
whole bunch of time-series in a new big model? Maybe by clever use of the
information in spmT-, con- and/or ResMS-images?
2) Another related point is: tests for the conjunction null across subjects
seem quite simple only with the single-subject's stats at hand: finding the
minimum t-value in each voxel across subjects and then thresholding the
resultant image. But what would be the correct number of degrees of freedom
for thresholding? Number of scans minus number of regressors (J-P) of one of
the single subject stats or J-P for a hypothetical huge fixed effects group
model?
3) A similar approach to the one outlined in 2) to test for the global null
conjunction simply doesn't want to cross my narrow mind. Can you imagine one?
Thanks a lot for any suggestions,
Thilo
--
Thilo Kellermann
RWTH Aachen University
Pauwelstr. 30
52074 Aachen
Tel.: +49 (0)241 / 8089977
Fax.: +49 (0)241 / 8082401
E-Mail: [log in to unmask]
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