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What else could cause this?
AdWords advertisers know that small changes in ad copy can cause significant changes in CTR. One savvy advertiser dropping out (or cutting back) and a couple new ones with lousy copy appearing can put the hurt on many sites. Do you have any idea how diverse (or narrow) your advertiser base is?
My topic usually has tons of ads to draw from but lately they seem to be showing a lot of the same ads page after page. I wonder if it's something about how the algo is choosing ads. For my site variety might be more important than just getting the top bidders over and over on page after page.
I am experiencing the same here too for 2 days now:
CTR down 50%
ecpm down 65%
visitors down 15%
earnings down 35%
And like you I have tons of advertisers and it is not likley that I can get affected by one or even 10 adverisers changing things or even dropping out.
So I am thinking there is a trend that needs more time to monitor before taking action.
People don't shop for my stuff on Fridays!
I track the best performing channels, and its not from any of them. CTR up slightly because I redid a site with rubbish click-through that was a quarter of my impressions, but the whole $4 a day its now bringing in wasn't what caused the upswing.
I'm as puzzled as the rest of you over it all, though possibly less distressed.
Something changed, thats for sure.
After looking at those stats I realize my present downturn is pretty minor next to that.
In the meantime try blocking every site that does not sell a real product or service in uk, us, canada with the preview tool. On all your pages. It might take a few hours monthly but it really does work like a charm.