Usage data representing the access of a set of resources on a network is accessed. The usage data is based at least in part on information received from client systems sent as a result of beacon instructions included with the set of resources. First and second sets of usage data representing access by client systems classified as a first type and a second type, respectively, are determined based on the accessed usage data. Counts of unique visitors accessing the network resources from each of the first and second types of client systems, based on the first and sets of usage data, respectively, are each determined. A total count of unique visitors accessing the network resources from the first and second types of client systems is determined based on data representing the usage overlap of devices of the first type with devices of the second type.