16. A document ranking method that uses a computer processor for calculation, comprising: calculating, using the computer processor, a content score of a first document with respect to each of at least one word contained in the first document; determining, using the computer processor, a contribution coefficient of the first document indicative of the first document's contribution value to a second document with respect to a common word included in the first document and the second document; calculating, using the computer processor, a contribution score of the first document based on the contribution coefficient of the first document; ranking, using the computer processor, the first document for the at least one word using the content score and the contribution score; and calculating, using the computer processor, an accumulation coefficient of the first document corresponding to a contribution by the first document to the contribution score of the first document to the second document, with respect to the common word, wherein the ranking comprises applying the accumulation coefficient to the content score and the contribution score, or applying the accumulation coefficient to the contribution score.
27. A non-transitory computer-readable medium storing a program for instructing a computer to perform the method of claim 16 .