Abstract: Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium for conducting an experiment for a campaign. A method includes: identifying a campaign including one or more content items and current selection criteria; receiving an indication of a proposed modification to be made to the campaign; storing the proposed modification as a draft campaign including the proposed modification in a first structure that includes the differences that are proposed to the campaign and a second structure that represents the campaign with the proposed modifications applied; determining a split to be applied as between using current campaign parameters or current selection criteria for a first portion of received traffic and using the proposed modification in the draft campaign for a second portion of the received traffic; and conducting an experiment based on using received traffic and in accordance with the split.