8. A computer readable storage device with an executable program stored thereon, wherein the program instructs a processor to perform: segmenting, by performing at least one of classifying and clustering, pieces of document data based at least in part on frequencies of occurrence of keywords in the pieces of document data, wherein the pieces of document data include a description in a natural language, the segmenting resulting in creating at least one document cluster and at least one keyword cluster; and obtaining a frequency distribution showing variation of the frequencies of occurrence of the pieces of document data by performing at least one of: a time-series analysis of frequencies of occurrence of pieces of document data containing individual keywords in at least one document cluster, and a time-series analysis of frequencies of occurrence of pieces of document data containing at least one keyword cluster.
8. A computer readable storage device with an executable program stored thereon, wherein the program instructs a processor to perform: segmenting, by performing at least one of classifying and clustering, pieces of document data based at least in part on frequencies of occurrence of keywords in the pieces of document data, wherein the pieces of document data include a description in a natural language, the segmenting resulting in creating at least one document cluster and at least one keyword cluster; and obtaining a frequency distribution showing variation of the frequencies of occurrence of the pieces of document data by performing at least one of: a time-series analysis of frequencies of occurrence of pieces of document data containing individual keywords in at least one document cluster, and a time-series analysis of frequencies of occurrence of pieces of document data containing at least one keyword cluster. 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.