Hi AFNI team,

I was using 3dclustsim to estimate the probability of false positive (noise-only) clusters. I estimated the smoothness values using 3dFWHMx (-acf). I obtained -acf values of 0.5128 3.8451 11.3839 which I inputted to the 3dclustsim.

The issue I face is that every time I run 3dclustim , I get different cluster size. Sometimes the values had a difference of more than 10. I was wondering if there is any issues with the way I am running 3dclustsim.

I have attached the screenshot of the output (N2 bi-sided) for reference.

Code used:

3dClustSim \
-mask $temp_path/MNI152_T1_2mm_brain_GM_02182017.nii.gz \
-acf 0.5128 3.8451 11.3839 \
-nodec \
-pthr 0.001 \
-athr 0.05 \
-prefix 3dClustSimOutput_censor_MR_6mm_ACF.txt

AFNI version=AFNI_20.2.16

I have also tried running 3dclustsim on a different data set with different smoothness values and yet I find cluster size differing every time I run 3dclustsim .