Occurrences: An occurrence of a 〈person, category〉 tuple is defined as a 4-tuple: where middle is a string surrounded by person and category.
a middle of an occurrence is not necessarily reliable, (Nguyen and Shimazu) proposed a method to retain reliable ones based on two criteria: repetition and diversity as follows: Repetition of a middle (repetition(middle)) is the number of times the middle appears between the person and category of (person, category) tuples of same person.