Hal Varian of Google, and his team spend most of thier time studying the AdWords auction and finding ways to make it more efficient. They share research study done on conversion rates by ad position: “on average, there’s very little variation in conversion rates by position for the same ad. For e.g. for pages where 11 ads are shown the conversion rate varies by less than 5% across positions. In other words, an ad that had a 1.0% conversion rate in the best position, would’ve about a 0.95% conversion rate in the worst position, on average. Ads above the search results’ve a conversion rate within ±2% of right-hand side positions. The bottom line: conversion rates don't vary much by position (emphasis added).”