TF-IDF values of two terms t i and t j , which occur in a test document d, are modified by reflecting the co-occurrence frequency value.
the terms with a high co-occurrence frequency value and a low category frequency value could have higher term weights as follows: where i) tw(t i ,d) denotes a modified term weight assigned to term t i , ii) cf denotes the category frequency, the number of categories in which t i and t j co-occur, and iii) is the maximum value among all co-occurrence frequency values.