R. Hausmann, B. van de Sande, and K. VanLehn. Intelligent Tutoring Systems, volume 5091 of Lecture Notes in Computer Science, page 636-645. Springer, (2008)
K. van de Sande, T. Gevers, and C. Snoek. Proceedings of the 2008 international conference on Content-based image and video retrieval, page 141--150. New York, NY, USA, ACM, (2008)