If you want the average per Journal title try this calculation:
WINDOW_SUM(AVG([Citations (C)]))/WINDOW_SUM(COUNTD([NCR ID]))
Add it to your table and make sure it's calculated using Pane-Down
Thanks Marten, very simple! I've been banging my head with this one for ages. I had actually tried something close to your solution but I don't think I was understanding the problem properly. I've managed to alter your solution and use it in other analyses already.
Thanks very much!!!