Y. Cui, and C. Penstein Rosé. Intelligent Tutoring Systems, volume 5091 of Lecture Notes in Computer Science, chapter 107, Springer-Verlag, Berlin/Heidelberg, (2008)
A. Nguyen, and W. Wobcke. Proceedings of the 10th international conference on Intelligent user interfaces, page 137--144. New York, NY, USA, ACM, (2005)