Hi,
You're on the right track. Add a covariate that is 1 for left and -1 for right. Assuming Ev1=group1 EV2=group2 and EV3=handedness, the
[ 0 0 1] contrast will test for when left>right
[0 0 -1] contrast will test for when right<left
No collinearity will occur if you limit yourself to these 3EVs and you can get at all inferences of interest.
Cheers,
Jeanette
On Mon, Mar 18, 2013 at 10:20 AM, Vincent
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Dear FSL experts,
When you create a design matrix using for example "design_ttest2 design 7 11" you will get a 2 by 18 matrix. The two columns both represent group membership, which are both needed in order to find out where group1 > group2 and where group2 > group1.
However, when I enter a dichotomous covariate of no interest (e.g. handedness), I do not need to create two columns. Creating one column would be sufficient to model out the effect of handedness. Is that correct?
If I would be interested for which voxels left > right, and when right > left, then I should create two columns, just like the group variable created by design_ttest2 (and change the the design.con file accordingly). Right?
Now if I consider the last scenario, where there are two columns for handedness, but that are entered as covariates of no interest, would that result in multicolinearity? How does randomise handles this?
Thanks,
Vincent