1. A computer-implemented method performed by a data processing apparatus, compromising: receiving reporting messages from user devices, each reporting message for a respective unique identifier, each unique identifier corresponding to a request for a resource sent from a user device, the reporting messages specifying display configurations of respective resources and display configurations of content item environments on the respective resources in which respective expandable content items were displayed; determining pairs of serving data log entries and reporting messages based on the unique identifiers, each serving data log entry specifying a predicted expansion direction and a unique identifier for the request, each of the pairs being a pair of a serving data entry and a reporting message having a matching unique identifier; and for each pair, determining an accuracy measure for the respective predicted expansion direction based at least in part on a comparison of the respective predicted expansion direction and the display configurations of the respective resource and content item environment, the accuracy measure representing an accuracy of the respective predicted expansion direction being a match for the respective resource and content item environment.