For example, we only observed 25 instances where a contradictory utterance was miscategorized as correct (compared to 200 "contradictory-pc incomplete" confusions), and so for many students this misclassification was never observed, and predictions based on this feature are not likely to be reliable.
we limited our features to those misclassifications that occurred at least twice for each student (i.e., at least 70 times in the entire corpus).