Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for creating experiment content distribution campaigns (“experiment campaign”) and allocating content in accordance with the experiment campaign. In one aspect, a control campaign or an experiment campaign are selected as an available campaign for providing content in response to receiving a content request. The selection can be a probability constrained selection where the probability with which the experiment campaign is selected is specified as a portion of a total number of requests received during an experiment period for which the experiment campaign is available to provide content. Content is provided in response to the request, where the provided content being content corresponding to the selected campaign.