If you are looking for the difference in the correlation between the groups then you have to use DODS and 0 0 1 -1. The way to understand it is that you have four regression coefficients and each means something. The way you have it set up in the fsgd file is that they are
1. group 1 intercept
2. group 2 intercept
3. group 1 cognition slope
4. group 2 coginition slope
Each coefficient gets multipled by its corresponding weight in the contrast and then the results are summed. So if you use 0 0 1 -1 you end up with contrast = g1cogslope-g2cogslope which is the difference you want

If you are just interested in whether the cog slope changes without regard to the group, then you would use 0 0 0.5 0.5 which means contrast = 0.5*g1cogslope + 0.5*g2cogslope which is the average of the slopes. In this case, using DOSS and 0 0 1 will give you the same results if there is no difference in the cog slopes between the groups



On 1/12/2023 8:48 AM, Agurne Sampedro Calvete wrote:

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Dear FreeSurfer experts, 

I have two groups and two time points and I would like to do a correlation analysis between a variable (cognition) and the change score in cortical thickness between the two time points (time2 minus time1). 

I would like to see if the correlation is different among each group, if there is an interaction with the group.

I have followed the paired t test instructions (MailScanner has detected a possible fraud attempt from "secure-web.cisco.com" claiming to be https://surfer.nmr.mgh.harvard.edu/fswiki/PairedAnalysis), but I am not sure about the contrast(s) I have to use. 

The paired-diff.fsgd file (second fsgd file) is the following:

GroupDescriptorFile 1
Title OSGM
Class Group1
Class Group2
Variables cognition
Input subject1pair Group1 0.546
Input subject1pair Group2 -0.243
...

How should it be the contrasts files to see the correlation of the variable cognition with the change score in thickness taking into account the groups? And should I use DODS or DOSS?

Using DODS:
0 0 0.5 0.5
 or 
0 0 1 -1 

I don't understand the difference between both contrasts. I think I should use the one of "0 0 1 -1" to see whether the cognition variable correlates with change in thickness differently according to the group. Is this correct?

Using DOSS:
0 0 1

Thank you very much in advance. 

Kind regards, 

Agurne



 



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