18. The computer program product of claim 17 , wherein the first and second word usage include first and second vectors, respectively, the first and second vectors having entries including words and a corresponding frequency metric; and wherein combining the first and second word usages comprises combining the first and second vectors.
18. The computer program product of claim 17 , wherein the first and second word usage include first and second vectors, respectively, the first and second vectors having entries including words and a corresponding frequency metric; and wherein combining the first and second word usages comprises combining the first and second vectors. 20. The computer program product of claim 18 , wherein the reference corpus is a taxonomy and the article is a node in the taxonomy.