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Adsense has a huge inventory of CPM ads to show and this is great for high traffic web pages. But these high traffic pages must dwarf the earnings data for the same CPM ad showing on a low traffic niche page. Why does this matter?
Because the CPM ads displace the better performing PPC ads (again for lower traffic, niche sites). If a CPM ad is showing, a PPC ad is not!
I really don't think anyone can legitimately argue that Google will not optimize ads to a website. CPC, CPM, Google knows what combination will result in the largest payout.
At the link above you can see my stats.
On August 16th, 2006 I asked Adsense to discontinue CPM ads for my account and they promptly did so. The change occurred on the 18th, And WOW:
For periods of 10, 30, and almost 60 days before and after this change, EARNINGS after Aug 18th are consistently UP approximately 40%.