15. The computer-implemented method of claim 12 further comprising: determining, with the computer system, a measure of confidence of ad scores for the document set; and adjusting, with the computer system, the document set score using the determined measure of confidence to generate a new document set score.
12. A computer-implemented method comprising: a) accepting, with a computer system including at least one computer, ad scores, wherein each of the ad scores indicates a value of a user selection of an ad; b) scoring, with the computer system, a document set including at least two documents using ad scores of the ads when the ads are served with the document set and ad scores of the ads when the ads are served across a collection of document sets to generate a document set score; c) accepting, with the computer system, a cost for an action with respect to an ad when the ad is served with a document of the document set; d) adjusting, with the computer system, the cost using the document set score to generate a modified cost; and e) billing, with the computer system, an advertiser associated with the ad, using the modified cost.