1. A method performed by data processing apparatus, the method comprising: receiving, at a data processing apparatus having a processor configured by executing instructions therein from a content distributor over a network, a set of candidate keywords specifying targeting keywords for one or more content items, each targeting keyword being a phrase to be matched for a content item to be eligible to be provided in response to a content item request; receiving, by the configured processor, for each of the targeting keywords, a probability distribution specifying likelihoods of specified user actions occurring in response to a content item being distributed, wherein the content item distributed based, at least in part, on the targeting keyword being matched by a content item request; including, in the probability distribution for each of the targeting keywords, a carry-over effect which represents the effect of distributing the content item based on the targeting keyword on the likelihood that the specified user actions will occur in response to the content item being subsequently distributed based on one or more other targeting keywords in the set of candidate keywords; selecting, by the configured processor, based on the probability distribution for each of the targeting keywords and the carry-over effect included in the probability distribution for each of the targeting keywords, a set of final keywords with which distribution of the one or more content items will be controlled, the set of final keywords being a plurality of keywords for which a likelihood of a specified outcome occurring exceeds a threshold likelihood, and the set of final keywords being a set of the candidate keywords for which a total cost of distributing the content items is below a specified value, wherein selecting candidate keywords to include in the set of final keywords based on the carry-over effect includes: determining, from the carry-over effect included in the probability distribution for a particular candidate keyword, that distributing the content item based on the particular candidate keyword increases the likelihood that the specified user actions will occur in response to the content item being subsequently distributed based on one or more other targeting keywords in the set of candidate keywords, adding the particular candidate keyword to the set of final keywords; and providing, by the configured processor over the network, an advertisement that is targeted using the final set of keywords.