3dTcorr1D vs 3dfim+

Dear AFNI experts,

I thought that 3dTcorr1D and 3dfim+ are essentially doing the same thing except for the latter
is working 3D+time data.

However, what I found was very little but important difference between the two functions.
When I did group analysis with 16 subjects (N=16)
3dTcorr1D is calculating Pearson Correlation with DF = 16,
but 3dfim+ seems to calculate regression coefficient with DF = 15.
With one regressor, I know they are the exactly same mathematically.
However, in terms of DF, they are different and the correlation coefficients are slightly different with different threshold and p-values.
Could you please explain the difference?
I am doing connectivity analysis, and have been using 3dfim+ but realized this difference…

Sungshin Kim

When I did group analysis with 16 subjects (N=16)

Do you have 16 or 17 subjects? Also, could you post your scripts?

Let me see the results more carefully…maybe I am wrong