Ryan Shaun Joazeiro de Baker Adriana M. J. B. de Carvalho Jay Raspat Vincent Aleven Albert T. Corbett Kenneth R. Koedinger Educational Software Features that Encourage and Discourage "Gaming the System". 475-482 2009 AIED http://dx.doi.org/10.3233/978-1-60750-028-5-475 conf/aied/2009 db/conf/aied/aied2009.html#BakerCRACK09