I noticed something very strange this past week. I was giving AdSense a fairly decent number of 336x20 impressions and was getting a decent CPM.
Yesterday, I decided to give them 728x90s as well, and the same number of 336x280s as always. My 728s got a decent CTR, but the CPM was crap. The 336s got the same CTR as always, but the CPM tripled!
I decided to stop giving AdSense 728s after just one day b/c of such a bad CPM, and my 336 CPM went way back down again.
What gives?? Should I keep the leaderboards up there even though their CPM sucks because doing so raises the CPM for my box ads?