S. Chien, Z. Vagena, D. Zhang, V. Tsotras, and C. Zaniolo. VLDB '02: Proceedings of the 28th international conference on Very Large Data Bases, page 263--274. VLDB Endowment, (2002)
N. Fuhr, and K. Großjohann. Proceedings of the 24th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, page 172-180. ACM Press, (2001)