K. Boyer, E. Ha, R. Phillips, M. Wallis, M. Vouk, and J. Lester. Proceedings of the Fourth Workshop on Innovative Use of NLP for Building Educational Applications, page 19--26. Stroudsburg, PA, USA, Association for Computational Linguistics, (2009)
P. Woodland, J. Odell, V. Valtchev, and S. Young. Proceedings of the 1994 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2, page 125-128. Adelaide, Australia, (April 1994)