Quality Score

Quality Score is a variable used by Google, Yahoo! (called Quality Index), and Bing that can influence both the rank and cost per click (CPC) of ads.

To determine the order in which ads are listed, each ad has the following formula run against it: bid * Quality Score. Ads are then listed in descending order based on the result of that equation. The exact weight of Quality Score versus bid has not been revealed by any of the major search engines, and each company has stated that they reserve the right to continually adjust their ranking methodologies.