12. A computer program product comprising a computer readable storage recording medium having computer code thereon, said computer code causing a computer to execute the operations of: extracting a plurality of common keywords associated with a piece of searching object data; storing said plurality of extracted common keywords into storage means; accepting a selected keyword selected by a user from said plurality of common keywords stored in said storage means; calculating exclusion efficiency indicating a level of exclusion efficiency of data that is not associated with said selected keyword for an individual common keyword of said plurality of common keywords stored in said storage means except for said selected keyword when said common keyword is specified as an exclusion keyword; and providing said exclusion efficiency to a user; wherein said calculating exclusion efficiency utilizes the selection keyword, the exclusion keyword, a searching object population set, and a calculation of a set of document data in which the selection keyword or the exclusion keyword appears in the searching object population set and determines a first set, a second set and a third set; wherein the first set comprises a set of document data in which the selection keyword appears in the searching object population set; wherein the second set comprises a set of document data in which the exclusion keyword appears in the first set; wherein the third set comprises a set of document data in which the exclusion keyword appears in the searching object population set wherein said calculating exclusion efficiency is based on a combination of a first term and a second term; wherein the first term comprises a ratio of a difference between a number of elements in a first set and a total number of elements in a second set, and the number of elements in the first set; wherein the second term comprises a ratio of a total number of elements in a third set and a total number of elements in the searching object population set; wherein the first term is adjusted by a first weight, and wherein the second term is adjusted by a second weight; wherein the sum of the first weight and the second weight is 1.
13. The program product according to claim 12 , wherein said calculating operation comprises causing the computer to calculate said exclusion efficiency, for an individual common keyword of said plurality of common keywords stored in said storage means except for said selected keyword, based on a first parameter indicating a level of exclusion efficiency in a set of data that is associated with said selected keyword when said keyword is specified as the exclusion keyword and a second parameter indicating a level of exclusion efficiency in a set of all of said searching object data when said keyword is specified as the exclusion keyword.