1. A method for retrieving a stored target document, comprising: conducting an initial search; receiving a user selection of a document from a result of the initial search; conducting an extended search based on the document that has been selected by the user; extracting a group to which the selected document belongs, from a plurality of groups each including associated documents that have been concurrently selected in response to previous retrieval requests; determining a frequency of co-occurrence between the selected document and the associated documents; identifying an associated document having a frequency of co-occurrence exceeding a predetermined threshold; designating the associated document as the target document; and extracting the target document.