1. An information retrieval system for of documents stored in a database, said documents being stored as document-keyword vectors generated from a predetermined keyword list, and said document-keyword vectors forming nodes of a hierarchical structure imposed upon said documents, said system comprising: a neighborhood patch generation subsystem for generating groups of nodes having similarities as determined using a hierarchical structure, said patch generation subsystem including a subsystem for generating a hierarchical structure upon said document-keyword vectors and a patch defining subsystem for creating patch relationships among said nodes with respect to a metric distance between nodes; and a cluster estimation subsystem for generating cluster data of said document-keyword vectors using said similarities of patches; and a graphical user interface subsystem for presenting said estimated cluster data on a display means.