R. Baeza-Yates, and E. Davis. Alternate track papers & posters of the 13th international conference on World Wide Web, page 328--329. New York, NY, USA, ACM Press, (2004)
S. Vassilvitskii, and E. Brill. SIGIR '06: Proceedings of the 29th annual international ACM SIGIR conference on Research and development in information retrieval, page 147--153. New York, NY, USA, ACM Press, (2006)