On Wed, Jan 9, 2013 at 9:08 PM, Jun Wang <[log in to unmask]> wrote:
> Dear spm experts,
> I have a question regarding to post hoc t test in spm. I have a 2X3 (group
> by task ) repeated design and got significant interaction when I used
> contrast [0 0 0 0 0 1 0 -1 -1 0 1].
>>>> The correct interaction contrast is an F-test. [0 0 0 0 0 1 -1 0 -1 0 0; 0 0 0 0 0 0 1 -1 0 -1 1]. This will tell you if there is an interaction between the task effects and group. From this, you could test for pairwise task difference interacting with your groups. With the post-hoc t-test of [0 0 0 0 0 1 0 -1 -1 0 1], you know the direction of the effect and don't need any further tests.
Then I want to do post hoc t test to
> tell the direction of interaction( e.g. group difference on task1). Should I
> extract ROI activity from significant cluster found in the interaction
> effect and do the simple t test outside spm or should I set different
> contrast in same SPM design. if it is the latter case, how should I set the
> contrast.
>>>> Testing the group effect of a single task in a repeated-measures design is not valid with the standard GLM. You'll need a 2-sample t-test. If you use GLM Flex, then you can test the group effect of individual tasks.
>
> thanks for your input
>
> Jun
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