Hi Mike,
Thank you for the quick reply. You have suggested 2 options and I want to make sure I understand how each would affect the output of the analysis.
>I would exclude the controls entirely from the input data set and the
>design matrix, since you are not interested in their correlations.
Is it necessary to remove the controls from the .mat file in order to obtain correct results, or will it work as I set it up with the controls padded with zeros? If they need to be removed, I assume that I would need to extract those images from the 4D all_FA_skeletonised image?
>Also, the contrast matrix for testing the correlation should be:
>0 0 1 0 (positive correlation for dg1)
>0 0 -1 0 (negative correlation for dg1)
>0 0 0 1 (positive correlation for dg2)
>0 0 0 -1 (negative correlation for dg2)
Just to check, this would perform independent correlations for each disease group and not group them together in a single correlation with the behavioral measure. If so, then I'm not sure I understand how adding the "exchangeability groups" would change the analysis....
>Lastly, you might want to consider setting up "exchangability groups",
>such that the permutations are limited to within the dg1 and dg2 groups,
>Respectively. (Or, alternatively, since you don't seem to be interested
>in any contrasts involving both dg1 and dg2, just run two completely
>separate analyses -- one involving the dg1 group, and the other
>involving the dg2 group).
This is exactly what I want to do (run two completely separate analyses -- one involving the dg1 group, and the other involving the dg2 group). Based on previous posts on 2 group correlations to a single EV, I thought that the first method would cover that. How would using an exchangeability group change the analysis? Is one a comparison of the means and the other a within-subject correlation? Would you be able to discuss how the statistics are performed in each case?
Thanks,
Laura
---------------------------------
Laura Danielian
Biomedical Engineer
National Institutes of Health
Building 10 CRC Room 7-5753
Bethesda, MD 20892
301-496-2168
________________________________
From: Michael Harms <[log in to unmask]>
Reply-To: FSL - FMRIB's Software Library <[log in to unmask]>
Date: Tue, 10 Nov 2009 12:31:49 -0500
To: <[log in to unmask]>
Conversation: [FSL] simple correlation
Subject: Re: [FSL] simple correlation
On Tue, 2009-11-10 at 16:59 +0000, Laura Danielian wrote:
> Hi,
>
> I appologize for the simple question, but statistics are not my strength. I
> have run the TBSS analysis and I want to investigate potential correlations
> between FA values within each voxel of the FA skeleton and a single
> behavioural measure. I have 3 groups (2 diseases, 1 control) and want to
> correlate values within each of the disease groups separately. I am not
> interested in the correlations for the control group. I have set up the
> following design.mat and design.con files (simplified to show only 2 subjects in
> each group). Are they appropriate for what I am trying to do? Rows 3-4
> represent the controls. The values in EV3 and EV4 were demeaned within the
> specific diagnosis.
>
> EV1(disease group 1) EV2(disease group 2) EV3(beh. meas. dg1) EV4(beh.
> meas. dg2)
> 1 0 0.26 0
> 1 0 -3.4 0
> 0 0 0 0
> 0 0 0 0
> 0 1 0 -4.3
> 0 1 0 4.68
>
> I have set up 4 contrasts to test both positive and negative correlations
> within each disease group:
>
> 1 0 1 0
> 1 0 -1 0
> 0 1 0 1
> 0 1 0 -1
>
> And will use the following randomise command:
>
> randomise -i all_FA_skeletonised -o tbss -m mean_FA_skeleton_mask -d
> design.mat -t design.con -n 5000 -c 3 -T2 -V -D
>
> Does this seem appropriate? Thank you so much for your help!
>
> Take care,
> Laura
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