Hi there,

I am wanting to use 3dFWHMx to calculate the smoothness of my data, so that I can run 3dClustSim. All of my preprocessing and analyses so far have been done in SPM, so I understand that I should use the ResMS.hdr file.

What I am not clear on is whether or not I should include the -2difMAD option for 3dFWHMx. Averaging across smoothing values calculated on each individual's ResMS images, I get quite high smoothing estimates (13, 14, and 13) without including this option, and more 'reasonable' smoothing estimates (7, 6, and 6) when I do include it (by 'reasonable', I mean more comparable to the smoothing kernel used in preprocessing, which was 8). The documentation for this option says it can be used when only using one input volume which has spatial structure, which the ResMS image has.

Any advice on whether I'm going about this the correct way would be very much appreciated. Thanks!