1. A method for searching pieces of document data using a search keyword, the pieces of document data having a correlation with the search keyword, the method comprising: receiving the search keyword from a user terminal to search an index database stored in an index creating computer; calculating as first vectors respective probabilities that each of the pieces of document data belongs to clusters, wherein each of the first vectors corresponds to one of the pieces of document data; calculating as a second vector, respective probabilities that the search keyword belongs to the clusters; calculating an inner product of each of the first vectors and the second vector, the calculated inner product being a first score of the corresponding piece of document data regarding the search keyword, wherein the inner product represents a scalar value; and acquiring a correlation value from a classification keyword set containing facet keywords and pieces of document data with the first score that is equal to or more than a predetermined threshold by multiplying a probability that a first word conceptually matching a facet keyword from the facet keywords occurs in the pieces of document data and a probability that a second word conceptually matching the search keyword occurs in the pieces of document data to generate a second score, and dividing a probability that both the first word and the second word occur in the pieces of document data by the second score, wherein the facet keywords represent a viewpoint of information using a plurality of attribute values as metadata and that are automatically selected by the index creating computer; and displaying a search result on the user terminal in descending order based on the correlation.
1. A method for searching pieces of document data using a search keyword, the pieces of document data having a correlation with the search keyword, the method comprising: receiving the search keyword from a user terminal to search an index database stored in an index creating computer; calculating as first vectors respective probabilities that each of the pieces of document data belongs to clusters, wherein each of the first vectors corresponds to one of the pieces of document data; calculating as a second vector, respective probabilities that the search keyword belongs to the clusters; calculating an inner product of each of the first vectors and the second vector, the calculated inner product being a first score of the corresponding piece of document data regarding the search keyword, wherein the inner product represents a scalar value; and acquiring a correlation value from a classification keyword set containing facet keywords and pieces of document data with the first score that is equal to or more than a predetermined threshold by multiplying a probability that a first word conceptually matching a facet keyword from the facet keywords occurs in the pieces of document data and a probability that a second word conceptually matching the search keyword occurs in the pieces of document data to generate a second score, and dividing a probability that both the first word and the second word occur in the pieces of document data by the second score, wherein the facet keywords represent a viewpoint of information using a plurality of attribute values as metadata and that are automatically selected by the index creating computer; and displaying a search result on the user terminal in descending order based on the correlation. 4. The method according to claim 1 , wherein the correlation value is acquired from a first data set of pieces of document data containing each keyword in a classification keyword set, a second data set of pieces of document data with respective third scores that are equal to or more than the predetermined threshold, a third data set of pieces of document data containing the search keyword, and a common data set of pieces of document data existing in both the first data set and the second data set.