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If it always turns out that the earning from Adsense drop in the second half of the month, simply swap the programmes and not show any google ads anymore but choose another programme.
It should be alright with the TOS if google ads are not shown at the same time.
This way, when adsense is "empty" of high-value clickthrough ads for your website, another programme could be used instead.
Or does it change every time?
The only think I know is, my earnings of Adsense have dropped the last 2 weeks now to something to an not very attractive level.
That will change in the future since there are competitors on the horizon, but not now.