Kelly Wauters Piet Desmet Wim Van Den Noortgate Acquiring Item Difficulty Estimates: a Collaborative Effort of Data and Judgment. Nominee for Best Paper Award. 121-128 2011 EDM http://educationaldatamining.org/EDM2011/wp-content/uploads/proc/edm2011_paper37_full_Wauters.pdf conf/edm/2011 db/conf/edm/edm2011.html#WautersDN11