X. Wei, and B. Croft. SIGIR '06: Proceedings of the 29th annual international ACM SIGIR conference on Research and development in information retrieval, page 178--185. New York, NY, USA, ACM Press, (2006)
G. Weikum. SIGMOD '07: Proceedings of the 2007 ACM SIGMOD international conference on Management of data, page 25--30. New York, NY, USA, ACM Press, (2007)