Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium, including a method for providing content. One or more content items are identified that are eligible to be presented in response to received requests in accordance with one or more selection criteria. One of the content items includes one or more parameters that can be varied. A range is identified over which a parameter of the one or more parameters is variable, and a subset of the range is identified, wherein the subset is less than the entire range. A request for content is received that is satisfied by the content item. The content item is prepared, including randomly selecting a value for the parameter that is within the subset of the range. At least one performance aspect associated with the prepared one content item is monitored.