Abstract: This paper describes timeout-based sessionizing mechanisms and topology-aware heuristics which now used to identify user sessions. The Sessionizing tools are based on heuristic rules and on assumptions about the site's usage,and therefore prone to error.The paper proposes a formal framework composed of a set of measures for the evaluation the accuracy of sessionizing tools.The different measures reflect the requitements of different web usage analysis applications.Experiment using the log data of a real web site shows the use of the measures.