Methods, systems, and apparatus for allocating advertising budgets. In one aspect, a method includes receiving a set of candidate keywords specifying targeting keywords for content items. Each targeting keyword can be a phrase to be matched for a content item to be eligible to be provided in response to a content item request. For each of the targeting keywords, a probability distribution specifying likelihoods of specified user actions occurring in response to a content item is received. Based on the probability distributions, a set of final keywords with which distribution of the content items will be controlled is selected, the set of final keywords being one or more keywords for which a likelihood of a specified outcome occurring exceeds a threshold likelihood. The set of final keywords is a set which a total cost of distributing the content items is below a specified value.