Google Adsense Chief Economist Hal Varian says that the placement of the Google Adsense advertising units will not make much difference in conversion rate.

This is a tricky question for several reasons. Since Google ranks ads by bid times ad quality, ads in higher positions tend to have higher quality and higher quality ads tend to have higher conversion rates. Thus you may see a correlation between auction position and conversion rates just due to this ad quality effect. However, the real question is how the conversion rate for the same ad would change if it were displayed in a different position.

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