Cognitive tutors are based on the Adapted Control of ThoughtRational (ACT-R) cognitive theory by John R. Anderson of Carnegie Mellon University. Cognitive tutors monitor student problem solving skills and map procedural knowledge to student actions. Skills are represented as production rules. Through model tracing the rules are matched to student actions to determine if the skill is correctly applied. Model tracing enables feedback and hints to be given. Knowledge tracing measures all relevant student skill levels over time. An example is the series of math tutors that were distributed commercially by Carnegie Learning, Inc., an offshoot of Carnegie Mellon University (Desmarais & Baker, 2012).