2. The computer-readable medium according to claim 1 , wherein the small category generating unit uses a hierarchical clustering to collect similar response records, the small category generating unit comprising: a hierarchy building unit to build a hierarchical structure of clusters by repetitively producing a new upper cluster from two lower clusters having the highest similarity at present which are selected from among the clusters each containing at least one response record, based on the definition of similarity; and a stop determining unit to determine when to stop the process of building the hierarchical structure by the hierarchical building unit, according to a predetermined stop condition, so as to finalize the hierarchical structure of clusters.
2. The computer-readable medium according to claim 1 , wherein the small category generating unit uses a hierarchical clustering to collect similar response records, the small category generating unit comprising: a hierarchy building unit to build a hierarchical structure of clusters by repetitively producing a new upper cluster from two lower clusters having the highest similarity at present which are selected from among the clusters each containing at least one response record, based on the definition of similarity; and a stop determining unit to determine when to stop the process of building the hierarchical structure by the hierarchical building unit, according to a predetermined stop condition, so as to finalize the hierarchical structure of clusters. 3. The computer-readable medium according to claim 2 , wherein: the hierarchy building unit evaluates the similarity between two clusters, based on a combination of first and second similarities thereof; the first similarity is defined as an absolute difference between a ratio of the response records created during a first half of the predetermined period and a ratio of the response records created during a second half of the predetermined period; and the second similarity is similarity in content between every two response records belonging to the two clusters in question.