The strength of concept links between two sentences, sim(si,sj), is in turn calculated as the weighted sum of the strength of each concept link in SL(si,sj).
	We use the data available at http://elib.cs.berkeley.edu/docfreq to get the term document frequency
	“Welcome to the Web Term Document Frequency and Rank site! Available from this site are the document frequency and rank of 31,928,892 terms found on 49,602,191 pages of the Web.”