Hello,
I apologize if the questions are trivial- I have checked the archives, but could not find anything on this.
I have run a t-test between two groups on cortical thickness, and used mri_glmfit-sim for pos, neg, and abs versions.
1. If my FSGD is organized as: Group 1 Group 2
and my matrix file is 1 -1
Would a blue result in an absolute version indicate that Group 1 < Group 2?
2. When running the command: "less" on "cache.th13.pos.sig.cluster.mgh", I am getting 1 significant cluster, however, when running the same command on the "absolute" version of this file, there is nothing significant. Do you know why this might be the case?
Much appreciated,
Arkadiy
On 07/08/2016 02:21 PM, Arkadiy Maksimovskiy wrote:
Hello,
I apologize if the questions are trivial- I have checked the archives, but could not find anything on this.
I have run a t-test between two groups on cortical thickness, and used mri_glmfit-sim for pos, neg, and abs versions.
- If my FSGD is organized as:
Group 1 Group 2
and my matrix file is 1 -1
Would a blue result in an absolute version indicate that Group 1 < Group 2?
Blue would mean that Group 1 < Group 2
- When running the command: "less" on
"cache.th13.pos.sig.cluster.mgh", I am getting 1 significant cluster, however, when running the same command on the "absolute" version of this file, there is nothing significant. Do you know why this might be the case?
A signed test (eg, pos or neg) has a different voxel-wise threshold than an unsigned test (eg, abs). By default, FS computes statistics in an unsigned manner, so when you specify a cluster-forming threshold with abs, that is the actual threshold that is used. When you use pos or neg, the cluster-forming threshold is made more liberal to account for the fact that you are doing a signed test. For a cluster that is all negative, using neg will result in a more liberal threshold and so a larger cluster which may be more significant.
Much appreciated,
Arkadiy
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