Highest CTR ad unit should be at the top

To make the maximum profit from Adsense, the adsense block with the highest CTR must be placed first in the HTML source. Added to 27 Adsense tips.

The Adsense guys have explained it in detail:

1) Create a channel for each ad placement. For example, if you have a leaderboard on the top of the page, a skyscraper on the right hand side, and another leaderboard on the bottom of the page, make sure you have a channel for all the top leaderboards, a second for the right skyscrapers, and a third for the bottom leaderboards.

2) Test the channels for at least a week to compare which position has the greatest clickthrough rate.

3) Make sure the the ad unit with the highest clickthrough rate is the first instance of the ad code that appears in the HTML. Since the first ad unit is always filled before the rest, you want to make sure that ad unit is located in the best placement on your page.