Hi Anna,

Please, see below:

On 16 November 2015 at 18:00, Anita van Loenhoud <[log in to unmask]');" target="_blank">[log in to unmask]> wrote:

Dear all,

I want to do a simple voxelwise correlation (both negative and positive) with education (i.e. only one predictor) in a group of 511 subjects. Education here is actually an ordinal variable, in which 1=lowest education and 7=highest education. My design.mat looks like this:

/NumWaves 1

/NumPoints 511

/Matrix

7

6

6

4

4

4

7

[etc]

My design.con looks like this:

/NumWaves 1

/NumPoints 2

/Matrix

-1

1

My randomise command looks like this:

randomise -i 4D.nii -o edu_correlation.randomise -d design.mat -t design.con -n 500 -D -T

I have a few questions:

1. Are my design.mat/con correct? Should I add PPheights=7 in design.mat and PPheights=1 in design.con?

Fine as is, no need for PPHeights.

2. Should I demean my data or not? I had a hard time finding that out in the manual/mailing list.

Yes, need -D since no intercept is modelled, and the interest is on the continuous variable.

3. Should I have added a GM explicit mask using –m (since I am only interested in gray matter voxels)? In principle, the images in the 4D.nii already only display GM voxels, so maybe a mask is not necessary?

If the input is already masked or already contains just what is of interest, no need for a mask.

4. I read that when you do a ‘one sample t-test’ you should use -1; does that apply to my data since I have one instead of two groups (I thought it doesn’t)?

A permutation test works fine for this design, even with just 1 group, with the continuous variable (so, no need for the -1).

All the best,

Anderson

Thanks a lot,

Anna