[O]ne important area that should not be overlooked is the placement of the ad. Google has put out a heat map study on ad placement and their performance. The colors fade from dark orange (strongest performance) to light yellow (weakest performance). All other things being equal, ads located above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page.

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