Evaluating web activity is disclosed. Initially, activity data for one or more resources on a network is received for a predetermined time period. The resources have been accessed by a plurality of client systems, and the activity data includes a unique identifier and a category of an accessing client system for each access to the one or more resources. Next, at least one persistent identifier of a client system within the activity data is identified. A subset of the activity data associated with the at least one persistent identifier is also identified. Based on the subset of the activity data, a total number of accesses to the one or more resources from the client system having the persistent identifier is determined. Finally, an estimated number of accesses to the one or more resources from the client system is determined if the persistent identifier persisted on the client system during the entire predetermined time period.