1. A method for determining the performance of a plurality of versions of a given advertisement, the method comprising: retrieving a first version of an advertisement and associated click through data; retrieving a second version of the advertisement and associated click through data; calculating a clickability score for the first version of the advertisement using the click through data associated with the first version of the advertisement; calculating a clickability score for the second version of the advertisement using the click through data associated with the second version of the advertisement; determining a difference in clickabilty scores between the first advertisement and the second advertisement; and modifying the clickability score associated with the first version of the advertisement based upon the difference in clickability scores.