A data processing system is described wherein use of resources in the system is metered. Metering evidence is generated which is difficult to forge and is, thus, relatively resistant to tampering. Such a system finds application, for example, in the field of metering the amount of time a person spends reading advertisement that are published on the World Wide Web. If reliable, such metering evidence may be used by the owner of the web server that contains the advertisement to bill the originator of the advertisement.