Dear Sofia
Sorry for the delay in replying. Your first two GLMs are correct - although note that you need a column of ones at the start of the design matrix (the intercept of the model), which I think is added automatically if you're using the batch GUI, but not if you're using Matlab scripting to call spm_dcm_peb directly.
Assuming the intercept is modelled as described above, I would be surprised if the two design matrices gave qualitatively different results. Do they lead to different conclusions?
Re: the order of subjects, no that's not important.
Best
Peter
-----Original Message-----
From: SPM (Statistical Parametric Mapping) <[log in to unmask]> On Behalf Of Sofia Amaoui
Sent: 29 April 2022 14:50
To: [log in to unmask]
Subject: [SPM] 3 groups comparison - PEB DCM
⚠ Caution: External sender
Dear Peter,
We are conducting a spDCM analysis and we are now in the group comparison step. We want to compare the connectivity strength of 3 different groups. We have been using the "https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fen.wikibooks.org%2Fwiki%2FSPM%2FParametric_Empirical_Bayes_&data=05%7C01%7Cpeter.zeidman%40UCL.AC.UK%7Cdfabfdd5bf3f493c21cf08da29e8b12f%7C1faf88fea9984c5b93c9210a11d9a5c2%7C0%7C0%7C637868376775906010%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=%2BOTAl8USm3vgDS6OqhLf7VWpagYw%2BnEdw3brPSsOnhg%3D&reserved=0(PEB)#First%20level%20analysis " web page. However, we still have a couple of questions:
1. How would be the correct design matrix if we want to compare:
(Group 1 vs. group 2) + (Group 1 vs. group 3) + (Group 2 vs. group 3)
2. So far, we compared group 1 vs. group 2 and group 1 vs. group 3 as follows:
[1 1
1 1
-1 0
-1 0
0 -1
0 -1]
Is that correct? When we swap the columns, the result remains the same. And if we change the positive values to negative values and vice versa, the results are the same, but inversed. So we are sure that so far, this is correct.
When we decided to change the second comparison for another one: instead of (group 1 vs. group 2) + (group 1 vs. group 3), we put (group 1 vs. group 2) + (group 2 vs. group 3):
[1 0
1 0
-1 1
-1 1
0 -1
0 -1]
Theoretically, the first regressor column is the same as before, however, by changing the second one, the results of the first comparison change. Why is this? Where is the error? Maybe you can only make a comparison of 3 groups, when at least 1 group remains the same in both comparisons? Therefore in this case, the group 2 should be defined in the same way?
3. Finally, I would like to ask you whether the order of the subjects is important. Should the subjects in each group be sequentially ordered? Or does it not matter as long as their group assignment is correct? An example:
[1 1
-1 0
0 -1
-1 0
1 1
0 -1]
Thank you so much for your help in advance.
Sofia,
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