Thanks, but I am having confusing results with this. When I run 3dFWHMx on my errts file

3dFWHMx -input SC01313_EGNG_SIEMENS_s1_Emo_RespXErr_SPMG3_errts_bucket+tlrc -ACF
# ACF model parameters for a*exp(-r*r/(2*b*b))+(1-a)*exp(-r/c) plus effective FWHM
0.961511 3.20408 7.81402 7.72251

Other errts files from this data set give similar results. Then from 3dClustSim I get:

3dClustSim -acf 0.961511 3.20408 7.81402 -mask MNI152_T1_3mm_brain_graymattermask.nii.gz
# CLUSTER SIZE THRESHOLD(pthr,alpha) in Voxels
# -NN 1 | alpha = Prob(Cluster >= given size)
# pthr | .10000 .05000 .02000 .01000
# ------ | ------ ------ ------ ------
0.050000 67.3 75.5 87.0 95.0
0.020000 30.6 34.2 39.2 43.0
0.010000 19.8 22.2 25.3 27.6
0.005000 13.8 15.5 17.8 19.5
0.002000 9.2 10.5 12.1 13.1
0.001000 7.0 8.0 9.3 10.2
0.000500 5.5 6.3 7.3 8.2
0.000200 4.0 4.6 5.5 6.1
0.000100 3.1 3.7 4.4 5.0

When I run it the old fashioned way, I get:

3dClustSim -fwhm 6 -mask MNI152_T1_3mm_brain_graymattermask.nii.gz
# CLUSTER SIZE THRESHOLD(pthr,alpha) in Voxels
# -NN 1 | alpha = Prob(Cluster >= given size)
# pthr | .10000 .05000 .02000 .01000
# ------ | ------ ------ ------ ------
0.050000 97.3 110.1 126.8 137.5
0.020000 44.9 50.7 58.1 63.1
0.010000 29.0 32.5 37.4 41.2
0.005000 20.1 22.9 26.6 29.2
0.002000 13.3 15.1 17.9 19.6
0.001000 10.0 11.6 13.7 15.2
0.000500 7.7 9.0 10.6 12.0
0.000200 5.5 6.6 7.9 8.9
0.000100 4.3 5.1 6.3 7.1

Why would my cluster sized be LESS stringent the newer way? Is there anything wrong with the way I am extracting the ACF parameters?

Thank you