Hi FSL list,
I'm a bit confused about the difference between the sigmasquareds and
varcope maps. Specifically, why is it necessary to have a different error
variance (varcope) for each contrast, as opposed to using the overall
residual (sigmasquareds)? How are the individual varcopes estimated (i.e.,
what makes error variance specific to one contrast and not another)?
Lastly, I'm comparing two groups in an unpaired t-test
(http://www.fmrib.ox.ac.uk/fsl/feat5/detail.html#UnpairedTwoGroupDifference)
with different values for the group membership variable, and two additional
contrasts to test each individual group mean (C3: 1 0 ; C4: 0 1). Shouldn't
the varcope for the contrast of each group mean (i.e., C3 and C4) be
different (they're not)? What is the best way to determine whether the error
variance does indeed differ for the two groups? Are these tests of the
individual group means valid when they're conducted within the context of
the unpaired t-test model?
Thanks,
Greg
___________________________________________________________________________
Greg Burgess, Ph.D.
Research Associate, Institute of Cognitive Science
University of Colorado - Boulder
Phone: 303-735-5802
Email: [log in to unmask]
Department of Psychology
Muenzinger Hall
UCB 345
Boulder, CO 80309
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