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)
H. Schütze, and J. Pedersen. Proceedings of the 4th Annual Symposium on Document Analysis and Information Retrieval, page 161--175. Las Vegas, USA, (1995)