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We can assume that Google is going to be experimenting like crazy trying out every combination they can imagine. I think this is going to be Google's secret lesson. Most webmasters will agree that sustainable revenue comes from non-stop experimentation. Keep the stuff that works, revise the stuff that doesn't. Rinse/repeat.
I got a email from google adsense team saying they are trying new content targeting algorithms and thats why some of my pages are showing PSA ads.
I've had one or two separate emails from them saying that as well. Both were replies to emails where I had asked why certain pages of mine which had been showing paid ads had started showing unpaid ads.
However, I think that part of the reply was a canned response, that they're regularly trying new algorithms and that in my case my issue is almost definitely related to their "negative content" detection scheme which is improperly classifying many of my pages as negative content due to text containing words like sex, rape and porn on a site that's the farthest thing from being adult content.
I didn't get a letter, so I'm wondering if they're just trying new stuff with a certain subset of people.
I doubt markus007 got an unprompted email out of the blue, but hopefully he'll clarify.
My server was down one of those days due to some huge power outage in California and perhaps I can blame the other three days on Mars or the heat.