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.