1. A method, comprising: receiving, by one or more data processing apparatus, a request to create an experiment campaign based on an existing campaign; receiving, by the one or more data processing apparatus through a communications network, data specifying an experiment parameter in the experiment campaign that differs from existing parameters of the existing campaign; storing, by the one or more data processing apparatus and in computer memory, the experiment campaign, comprising storing a set of experiment difference data that i) excludes at least one of the existing parameters that is also a parameter of the experiment campaign, and ii) specifies a difference between the experiment parameter and the existing parameters; receiving, by the one or more data processing apparatus through the communications network, a request to serve content to be presented on an electronic document requested by a user device executing an application at the user device, wherein the request includes a network address identifying the user device; generating, after receiving the request for content by the one or more data processing apparatus, a set of experiment parameters for the experiment campaign using the at least one of the existing parameters from the existing campaign and the stored experiment difference data; and in response to the request, transmitting, to the user device through the communications network, content that is integrated into a presentation of the electronic document by the user device, wherein the content is transmitted to the user device based on the set of experiment parameters.