M. Pasca, B. Durme, and N. Garera. CIKM '07: Proceedings of the sixteenth ACM conference on Conference on information and knowledge management, page 485--494. New York, NY, USA, ACM, (2007)
U. Lee, Z. Liu, and J. Cho. WWW '05: Proceedings of the 14th International World Wide Web Conference, page 391--400. New York, NY, USA, ACM Press, (2005)