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Dear FS experts,
Instead of using mri_glmfit-sim, I am trying to implement a customized multiple comparison correction algorithm using permutation. Before I implement my own, I want to make sure my permutation idea is correct. So I was looking at how mri_glmfit-sim does the permutation. The link here http://freesurfer.net/fswiki/FsTutorial/MultipleComparisonsV6.0Perm has a simple description for how to do permutation, but I don't quite understand the 1st step: Permute the design matrix. To me, permute the design matrix means permute the matrix rows here, but still hard to understand why permuting matrix rows does the trick. Anyway, I will not use any design matrix in my customized implementation, so it does not matter for now. My problem can be described as follows: if I have a ROI (a label file) on fsaverage, and I have a test subject and a group of control subjects whose thickness values on every vertex in this label are all known. (The test subject and control subjects are all using fsaverage reference space). I want to compare this test subject's thickness within the ROI to a control group of subjects (within the same ROI). This is a multiple-comparison problem, so I want to use permutation to get less FP rate. My question is: How do I permute the test subject's points and control subjects' points in ROI? My understanding is that: for each point in the label, I randomly re-assign all (1+n) values from (1+n) subjects to these (n+1) subjects (where n is the number of subjects in control group). And when all points in the label are done, this is only 1 permutation. I will need at least 1000 times of permutation to get the comparison statistics. Is my understanding right? Thank you! Xiao
On 7/21/2020 1:12 AM, Xiaojiang Yang wrote:
External Email - Use Caution
Dear FS experts,
Instead of using mri_glmfit-sim, I am trying to implement a customized multiple comparison correction algorithm using permutation. Before I implement my own, I want to make sure my permutation idea is correct. So I was looking at how mri_glmfit-sim does the permutation. The link here http://freesurfer.net/fswiki/FsTutorial/MultipleComparisonsV6.0Perm%C2%A0has a simple description for how to do permutation, but I don't quite understand the 1st step: Permute the design matrix. To me, permute the design matrix means permute the matrix rows here, but still hard to understand why permuting matrix rows does the trick.
This is pretty standard in permutation. I think Tom Nichols has some basic tutorials on how permutations work.
Anyway, I will not use any design matrix in my customized implementation, so it does not matter for now. My problem can be described as follows: if I have a ROI (a label file) on fsaverage, and I have a test subject and a group of control subjects whose thickness values on every vertex in this label are all known. (The test subject and control subjects are all using fsaverage reference space). I want to compare this test subject's thickness within the ROI to a control group of subjects (within the same ROI). This is a multiple-comparison problem, so I want to use permutation to get less FP rate. My question is: How do I permute the test subject's points and control subjects' points in ROI?
Not sure what you mean by in the ROI. Are you trying to permute across space? That generally does not work because the points are not exchangeable across space.
My understanding is that: for each point in the label, I randomly re-assign all (1+n) values from (1+n) subjects to these (n+1) subjects (where n is the number of subjects in control group). And when all points in the label are done, this is only 1 permutation. I will need at least 1000 times of permutation to get the comparison statistics. Is my understanding right?
Not sure. You cannot discriminate between the groups when you are doing permutation
Thank you! Xiao
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Xiaojiang Yang