hi Christine

my understanding from the following excerpts from the help is that it is more like 3dmerge, in that specifying -FWHM 6 does not mean that the resulting data will have 6mm smoothness but rather something larger based on the intrinsic smoothness of the data before it is processed by 3dBlurInMask:


* Works iteratively, similarly to 3dBlurToFWHM, but without
the extensive overhead of monitoring the smoothness.

* If the original FWHM of the dataset was 'S' and you input a value
'F' with the '-FWHM' option, then the output dataset's smoothness
will be about sqrt(S*S+F*F). The number of iterations will be
about (F*F/d*d) where d=grid spacing; this means that a large value
of F might take a lot of CPU time!

hope this helps (and that my interpretation of the help is correct!)

James