18. The computer-implemented information processing of claim 13 , wherein the word is a first word, the word score is a first word score, and the method further comprises the steps of: extracting a second word from the document; calculating a second word score for the second word; and calculating the sentence score for the sentence on the basis of the first word score and the second word score.
13. A computer-implemented information processing method comprising the steps of: receiving instructions from a user to search for a document; searching for the document in a storage; storing the document in a memory; classifying a document into a first category and a second category; extracting a word from the document; determining a first total number of documents in the first category; determining a second total number of documents in the second category; determining a first number of documents as a first count of the documents in the first category that contain the word; determining a second number of documents as a second count of the documents in the second category that contain the word; calculating, by a processor: a first ratio of the first total number of documents and the first number of documents; a second ratio of the second total number of documents and the second number of documents; and a word score for the word using a mathematical product of the first ratio and the second ratio, wherein the word score is higher, when the word appears less frequently in the category; extracting a sentence from the document; calculating a sentence score for the sentence on the basis of the word score; and displaying the sentence on a display, when the sentence score exceeds a threshold.