1. A computer-implemented method comprising: receiving selection criteria for a campaign, the selection criteria including a plurality of keywords that control distribution of content items associated with the campaign; assigning, by a computer, each of the selection criteria to one or more sets of topic clusters, wherein at least some of the selection criteria are assigned to multiple topic clusters; determining, by a computer and for pairs of selection criteria in one of the topic clusters, a measure of similarity between the topic clusters to which each selection criteria in the pair was assigned; identifying, by a computer and as related pairs of selection criteria, the pairs of selection criteria for which the measure of similarity meets a threshold; creating a new keyword cluster based on the related pairs, the new keyword cluster including fewer than all of the keywords in the received selection criteria, the creating comprising: identifying a first selection keyword and a second selection keyword that are included in one of the related pairs; and including each of the first selection keyword and the second selection keyword in the new keyword cluster; and creating, by a computer, a new group for the campaign, the new group specifying at least one content item that is selected for distribution using keywords in the new keyword cluster.
2. The method of claim 1 , wherein assigning each of the selection criteria to one or more sets of topic clusters comprises assigning each selection keyword in a set of selection keywords to one or more topic clusters corresponding to predetermined classifications.