Umarani Pappuswamy Dumisizwe Bhembe Pamela W. Jordan Kurt VanLehn A Multi-Tier NL-Knowledge Clustering for Classifying Students' Essays. 566-571 2005 conf/flairs/2005 FLAIRS Conference db/conf/flairs/flairs2005.html#PappuswamyBJV05 http://www.aaai.org/Library/FLAIRS/2005/flairs05-093.php