14. The computer readable storage device according to claim 11 , wherein acquiring a time series of each keyword cluster further includes applying a filter to a data set of pieces of document data.
11. The computer readable storage device according to claim 8 , wherein the program further instructs the processor to perform: receiving a search query; narrowing down a set of pieces of document data by the search query; acquiring, from a time series of a keyword in the narrowed set of pieces of document data, a time series of each keyword cluster in the narrowed set of pieces of document data; and performing a frequency analysis of the time series of the keyword cluster.