hi Gang

thanks a lot for that clarification. I have noticed that SPM does things one-sidedly, in that if you set the uncorrected p to .001 you get the t-value that corresponds to p < .002 on the AFNI viewer's slider. This makes sense in the terms of a one-sided vs. two-sided test.

My question is then, if you run AlphaSim or 3dClustSim with an uncorrected p of .001, and it says that you need, say, 118 voxels for an alpha of .05, should the 3dmerge command typically use the value given on the AFNI slider for that p as the -1thresh option (in this case, 4.07--the 2-sided value for t(15)) or 3.73 (the one-sided value). As in, what is typically expected of the user? I feel as though the two-sided value is appropriate since in AFNI the contrasts are typically output as two-sided (i.e., if I contrast two conditions I get both positive and negative clusters) whereas in SPM the contrasts have to be run in both directions. But maybe I am confusing my terminology.

hope this is clear. thanks so much for your help.

James