E. Voorhees. Proceedings of the 16th annual international ACM SIGIR conference on Research and development in information retrieval, page 171--180. New York, NY, USA, ACM, (1993)
M. Sanderson. Proceedings of the 17th annual international ACM SIGIR conference on Research and development in information retrieval, page 142--151. New York, NY, USA, Springer-Verlag New York, Inc., (1994)