J. Beck, K. min Chang, J. Mostow, and A. Corbett. Intelligent Tutoring Systems, volume 5091 of Lecture Notes in Computer Science, page 383-394. Springer, (2008)
K. min Chang, J. Beck, J. Mostow, and A. Corbett. Intelligent Tutoring Systems, volume 4053 of Lecture Notes in Computer Science, page 104-113. Springer, (2006)