So I am a little confused, then. If i have individual subjects that were run with two EVs contrasts (1,0) (0,1) (1,1), do I have to go back and rerun each individual subject with contrasts (1,-1)(-1,1) in order to get group differences or can I manipulate these in some way?

On Tue, Sep 20, 2011 at 11:14 AM, Michael Harms <[log in to unmask]> wrote:

Group contrasts (and statistics) should be calculated from the
appropriate copes, not done "post-hoc" by subtracting z-maps.

See
http://www.fmrib.ox.ac.uk/fsl/feat5/detail.html#UnpairedTwoGroupDifference
which provides a clear example of how to proceed.

cheers,
-MH

On Tue, 2011-09-20 at 11:07 -0400, Leslie Engineering wrote:
> does it make a difference if instead I have one group that went
> through a single paradigm, EV. If EV is made up of two separate
> stimuli blocks and I want to separate them into  EV1 and EV2, can I
> run a single group average using EV1 and a single group average using
> EV2 then subtract the z scores for an activation difference between
> the two stimuli?
>
>
> thanks so much
>
> On Tue, Sep 20, 2011 at 10:08 AM, Leslie Engineering
> <[log in to unmask]> wrote:
>         Hello-
>
>
>         I ran two separate simple group average analysis. Both groups
>         underwent the same fmri scan sequence. Instead of rerunning a
>         group analysis and included all subjects from both groups and
>         setting up contrasts for group differences (1,-1)(-1,1), is it
>         statistically sound to simply subtract their zscores?
>
>
>
>
>
>