Initially, a rough estimation of the significance of a word is given by its frequency in the document set.
algorithm 1: Hierarchical Tree Construction 1: Sort the identified key words by their frequency in the document set in descending order, denoted as T = {t 1 , t 2 ,…, t n } 2: For each t i , i from 1 to n, find the most relevant word t j from all the words before t i in T, as Here the relevance of two words is calculated by their PMI, i.e.