K. Tzeras, and S. Hartmann. 16th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, page 22-34. Pittsburgh, PA USA, ACM, (1996)
N. Fuhr, and P. Muller. 10th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, page 13-18. New Orleans, Louisiana, United States, ACM Press, (1987)