Methods, and systems, including computer programs encoded on computer-readable storage mediums, including a method for determining accuracy measures for predicted expansion directions for content item environments. The method includes, identifying a predicted expansion direction for the content item environment; generating a serving data log entry for the request specifying the predicted expansion direction and a unique identifier for the request; providing an expandable content item having the predicted expansion direction and response data specifying the unique identifier; receiving reporting messages specifying the display configurations of the resources and content item environments; determining pairs of serving data log entries and reporting messages based on the unique identifiers, each pair having a matching unique identifier; and determining an accuracy measure for the respective predicted expansion direction based on a comparison of the respective predicted expansion direction and the display configurations of the respective resource and content item environment.