https://www.google.com/adsense/support/bin/answer.py?answer=17955

isi post As a rule of thumb, wider ad formats tend to outperform their taller counterparts, due to their reader-friendly format. Readers absorb information in thought units (that is, several words at a time). The wider format lets them comfortably read more text at a glance without having to skip a line and return to the left margin every few words as they would be forced to do with a narrower ad. The wider ad format also lessens the likelihood of readers leaving the ad unit altogether. If positioned well, these ad formats can increase your earnings significantly. The formats we've found to be the most effective are the 336x280 Large Rectangle, the 300x250 Medium Rectangle, and the 160x600 Wide Skyscraper. Keep in mind that while these ad formats typically perform well, you should use the format that best complements your pages. Tip for making the most of a little ad space: Try one of our streamlined link unit formats, shown below. These formats are lean, mean, and versatile enough to fit in locations normal ads don't. info https://www.google.com/adsense/support/bin/answer.py?answer=17955
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