It's Official, First Adsense Block On Page Pays Most
The adsense team have just confirmed something which most Adsense publishers have known, Or at least suspected, For some time. And that is that the adsense adblock that appears first will contain higher paying adverts.
Currently, the first ad unit on a page always shows the top ads that win the ad auction. Also, if there aren't enough ads in our ad inventory to fill all of the ad units on a page, the first ad unit on the page will display ads first. This is why I recommend using custom channels to determine which of your ad units has the highest CTR, and then placing that ad unit first in the HTML code.
When the Adsense team say "the first ad unit on the page" they mean the adblock that appears closest to the top in your source code, Not the first to appear on the page. It's possible that the adblock that loads first will actually be the last to receive adverts if it is near the bottom of your source code or positioned using a DIV.
So like the adsense team say, First impressions count. Make sure that the adverts closest to the top of your source code are the adverts that receive the highest number of clicks, You can find this CTR by setting up a custom channel for each of the adblocks that appear on the page. So if your high paying adblock is receiving less clicks that the others you may have to rejig your source code.
The rule is, The adsense adblock that appears first in your source code will contain the highest paying adverts, So you should position this in a high CTR area of your page