1. A method performed by data processing apparatus, the method comprising: creating, by a data processing apparatus, an initial experiment campaign having initial parameters that match a set of campaign parameters for a control campaign; receiving, by a data processing apparatus, from an advertiser computer, experiment parameters that modify the initial experiment campaign to create an experiment campaign that differs from the control campaign; generating, by a data processing apparatus, experiment difference data associated with the experiment campaign based on differences between the experiment parameters for the experiment campaign and the set of campaign parameters for the control campaign, the generated experiment difference data excluding one or more experiment parameters that match a control campaign parameter in the set of campaign parameters; receiving, by a data processing apparatus, a request for content from a user device; and providing, in response to the request and for presentation at the user device, content from the experiment campaign, the content being provided based on both the control campaign parameters that match the one or more excluded experiment parameters and the experiment difference data associated with the experiment campaign.