5. The method of claim 2 in which calculating the tf value of the constituent term further comprises: for each confirmed valid search result for the queries from which the constituent term is extracted: dividing a number of times the constituent term appears in the search result by a length of the search result to obtain a relative term frequency; and applying the relative term frequency to the tf value of the constituent term.
2. The method of claim 1 in which calculating a tf-idf value of each extracted constituent term further comprises: calculating an inverse document frequency (idf) value of the constituent term in relation to the corpus of documents; calculating a term frequency (tf) value of the constituent term; and determining the tf-idf value of the extracted constituent term based on the idf value of the constituent term and the tf value of the constituent term.