AFNI Message Board

Dear AFNI users-

We are very pleased to announce that the new AFNI Message Board framework is up! Please join us at:

https://discuss.afni.nimh.nih.gov

Existing user accounts have been migrated, so returning users can login by requesting a password reset. New users can create accounts, as well, through a standard account creation process. Please note that these setup emails might initially go to spam folders (esp. for NIH users!), so please check those locations in the beginning.

The current Message Board discussion threads have been migrated to the new framework. The current Message Board will remain visible, but read-only, for a little while.

Sincerely, AFNI HQ

History of AFNI updates  

Previous Message Next Message
Message List New Topic Print View
|
gang
September 29, 2014 10:27AM
> 3dttest++ -prefix blabla.nii -setA group1/*.nii -setB group2/*.nii
> 3dcalc -a blabla.nii[0] -expr a -prefix map_mean.nii
>
> Is it correct?

The 3dttest++ command gives you the connectivity difference between the two groups and the associated t-statistic. I'm not so sure about the purpose of the 3dcalc command.

> If so, why if I reverse group A and B
>
> 3dttest++ -prefix blabla.nii -setB group1/*.nii -setA group2/*.nii
> 3dcalc -a blabla.nii[0] -expr a -prefix map_mean.nii
>
> I obtain different results/maps?

There is no point reversing the order of the two groups. By default the t-test is two tailed in AFNI, so the first 3dttest++ command should give all you want. If you want to sort out the directionality, the positive or negative sign in the output should be good enough for that purpose. If you think that one-tailed thresholding is justified, you can simply divide the p-value underneath the slider bar on the AFNI GUI by 2.

Gang
Reply Quote
RSS
Subject Author Posted

t-test - 3dttest++

Marco P September 29, 2014 09:19AM

Re: t-test - 3dttest++

gang September 29, 2014 10:27AM