What's in a Session Tracking Individual Behavior on the Web

Mark R. Meiss1,2, John Duncan1, Bruno Gonçalves1, José J. Ramasco3 and Filippo Menczer1,3
1School of Informatics, Indiana University, Bloomington IN, USA.
2Advance Network Management Lab, Indiana University, Bloomington IN, USA.
2Complex Networks and Systems Lagrange Laboratory, CNLL, ISI Foundation, Turin, Italy.

(June 2009)

We examine the properties of all HTTP requests generated by a thousand undergraduates over a span of two months. Preserving user identity in the data set allows us to discover novel properties of Web traffic that directly affect models of hypertext navigation. We find that the popularity of Web sites-the number of users who contribute to their traffic-lacks any intrinsic mean and may be unbounded. Further, many aspects of the browsing behavior of individual users can be approximated by log-normal distributions even though their aggregate behavior is scale-free. Finally, we show that users' click streams cannot be cleanly segmented into sessions using timeouts, affecting any attempt to model hypertext navigation using statistics of individual sessions. We propose a strictly logical definition of sessions based on browsing activity as revealed by referrer URLs; a user may have several active sessions in their click stream at any one time. We demonstrate that applying a timeout to these logical sessions affects their statistics to a lesser extent than a purely timeout-based mechanism.