1. A method performed by data processing apparatus, the method comprising: accessing data specifying a plurality of search queries; identifying, by one or more data processing apparatus and based on at least some of the search queries, a first content distribution campaign in which distribution of at least one content item provided by a first content provider is conditioned on a distribution parameter matching one of the search queries and a second content distribution campaign in which distribution of at least one content item provided by a second content provider different from the first content provider is conditioned on a distribution parameter; determining, by one or more data processing apparatus and based on one or more distribution parameters of the first content distribution campaign matching one or more distribution parameters of the second content distribution campaign, that the first content distribution campaign and the second content distribution campaign have at least a specified level of similarity to each other; identifying, by one or more data processing apparatus, a particular distribution parameter of the first content distribution campaign that is not included in the one or more distribution parameters of the second content distribution campaign; determining, by the one or more data processing apparatus and for the particular distribution parameter, a first number of users that submitted a search query matching the particular distribution parameter over a first time period; determining, by the one or more data processing apparatus and for the particular distribution parameter, a second number of users that submitted a search query matching the particular distribution parameter that is matched by a particular search query over a second time period that differs from the first time period and occurs after the first time period; determining, by the one or more data processing apparatus and for the particular distribution parameter, a trend score based on comparing the first number of users and the second number of users; determining, by the one or more data processing apparatus, that the trend score is an increasing trend score based on the second number being greater than the first number; and providing, by the one or more data processing apparatus and based on the trend score and on the specified level of similarity between the first content distribution campaign and the second content distribution campaign, suggestion data suggesting the particular distribution parameter as an additional distribution parameter for the second content distribution campaign.