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They have to be different because you processed the data differently. 3dTcat simply concatenates all the scanning sessions together while '3dcalc ...-expr '(a+b+c+d+e+f)/6' generates the average of six data volumes.

There are different ways to preprocess the data, and it depends on what questions the user wants to answer. Refer to HowTo1 and HowTo5 for more details:

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[afni.nimh.nih.gov]

Gang