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?

I confirmed this! I gave AdSense another day of leaderboards, and this time eCPM climbed four times what I get without leaderboards ... yet the leaderboards give me a VERY poor CPC! (I have leaderboards and boxes on their own channels - whether leaderboards are enabled or not directly influences the CPM of the box ads fourfold!)

Call it a hunch, but here is what I think. Google now has to compete with Yahoo!'s Publisher Network. What a lot of people are doing is testing out Yahoo!, and giving half their impressions to each network. Now Google knows that I am capable of delivering X impressions. So when I give them a lot less, they assume that I'm leaving them to give my impressions to another company, and therefore lower the CPM rate ... In other words, they'll just give me a high CPM if I give them everything I have. See what I'm saying?

Dani, I have noticed this. However, I've also noticed the opposite at some points in time (well actually, displaying half the number of ads but getting the same payout as you would with more ads (and clicks)).

I think it has to do with the bidding difference between CPM and CPC buys. If you add a banner (which also ads a lot more CPM opportunities into the bidding), I think the CPC people will have to bid more.

However, I just added a 125x125 button which displays on every page of the site (the other adsense ads only appear on the main page and as defaults for tribal fusion) and my eCPM for the front page ads jumped to 3 times what it was yesterday. I didn't even know that there were traditional Google CPM ads of that size, but I realized you can place a CPM bid for a normal sized individual Adsense ad too (I think).

Yes, you can do a CPM bid on text campaigns or banner campaigns.