We're using AlphaSim to estimate a cluster extent threshold for our group maps, and a reviewer for one of our articles suggested that we must used 3dFWHMx to estimate the true smoothness in X, Y, and Z rather than just putting the size of the blur we applied to the data (using the -FWHM option). We applied a 3.5mm FWHM blur to the data using 3dmerge, but our actual values for smoothness in the group map is much larger (x = 7.2, y = 6.97, and z = 5.79). Obviously, when we run AlphaSim with these updated values, the output indicates that we must use a much larger cluster size (21 voxels) to achieve an alpha < .05.

Our question is how should we handle this when we have multiple group maps that need to be corrected for multiple comparisons? For example, we have two conditions that are compared against an active baseline condition. So do we need to estimate the smoothness for each map individually, and then use the smallest cluster size that achieves alpha < .05 for all maps? We would prefer to apply the same cluster size across maps, rather than using unique cluster sizes for each map.

Any advice would be appreciated.

Michael