Hi Alfonso,
 
Thanks very much.  This worked, and as I was exploring the clusters, I was prompted to enter contrast weights in a contrast manager, which had 4 columns.  I had two groups and two sources selected.  Would the first two columns refer to the two groups, and the last two columns refer to the two sources?  And if I wanted to compare the patient group to the control group across the two sources, I would enter 1 -1 0 0?
 
Thanks again,
Mary