Devon Barrow Antonija Mitrovic Stellan Ohlsson Michael Grimley Assessing the Impact of Positive Feedback in Constraint-Based Tutors. 250-259 2008 Intelligent Tutoring Systems http://dx.doi.org/10.1007/978-3-540-69132-7_29 conf/its/2008 db/conf/its/its2008.html#BarrowMOG08