1. A method performed by data processing apparatus, the method comprising: receiving, by a data processing apparatus, a request to create an experiment campaign corresponding to a control campaign, the control campaign including campaign parameters with which allocation of content corresponding to the control campaign is controlled; creating, by a data processing apparatus, an initial experiment campaign having initial parameters that match a set of campaign parameters for the control campaign; receiving, by a data processing apparatus, experiment parameters that modify the initial experiment campaign to create the experiment campaign that corresponds to the control campaign, the experiment parameters being used to adjust at least one parameter from the initial parameters, the experiment parameters including data specifying an experiment period and a portion of content requests received during the experiment period for which the experiment campaign is available to provide content; activating, by a data processing apparatus, the experiment campaign in response to a start condition, the start condition being a specified condition that upon detection causes the experiment campaign to be activated, wherein activation of the experiment campaign causes the experiment campaign to be available, as specified by the experiment parameters, to provide content in response to content requests; computing, by a data processing apparatus and for content requests received during the experiment period, performance measures for the control campaign and the experiment campaign; and providing, by a data processing apparatus, data that causes presentation of the performance measures corresponding to the control campaign and the experiment campaign, the data further causing presentation of variation identifiers that represent variations between performance measures for the control campaign and the experiment campaign that exceed a variation threshold.