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My hypothesis is that when they rolled out this change, they hit the RESET button on the targeting statistics and its going to take time for googles engine to re-learn what ad's to put on what sites again.
Everything's back to normal now (and there are arrows all over my sites) but I'm still wondering if they blipped the whole network while they were making these changes.
Play_Bach - high invalid click count? Is this a guess on your part or is there some google number im not aware of that shows how many invalid clicks you are getting?