Google provided a very detailed explanation today of how the average position metric works and what it really means. I thought it was interesting that Google mentions that they have discovered that conversion rates don’t vary much with the position of the ad on the page.
Google specifically mentions, “An ad in a more prominent position on the page will tend to get both more clicks and more conversions than an ad in a lower position, but the conversion rate (conversions/clicks) will tend to be about the same for the two positions. In summary, average position is a popular metric, but don’t rely on it alone as a measure of performance. The metrics that really matter are clicks, costs, and conversions.”
You can read the complete article here: Understanding the Average Position metric