17. The computer-implemented method of claim 12 further comprising: determining, with the computer system, the accepted ad scores, wherein the act of determining the accepted ad scores includes, for each ad served with a document of the document set for which a value of an action with respect to the ad is tracked, i) for each document set A) determining a score for the ad on the document of the document set, and B) combining the determined score for the ad on the document with previously determined scores for the ad on other documents of other document sets to generate a combined score for the ad, and ii) determining a new ad score across a collection of document sets using the combined score for the ad.
17. The computer-implemented method of claim 12 further comprising: determining, with the computer system, the accepted ad scores, wherein the act of determining the accepted ad scores includes, for each ad served with a document of the document set for which a value of an action with respect to the ad is tracked, i) for each document set A) determining a score for the ad on the document of the document set, and B) combining the determined score for the ad on the document with previously determined scores for the ad on other documents of other document sets to generate a combined score for the ad, and ii) determining a new ad score across a collection of document sets using the combined score for the ad. 18. The computer-implemented method of claim 17 wherein the new ad score across the collection of document sets is an average score.