Patent ID: 8065321

Claim:
A document retrieval apparatus, comprising: a storage device configured to store correspondence information indicating the correspondence between a registered symbol set and a registered document, wherein the registered symbol set is extracted from a plurality of symbols representing the registered document, the plurality of symbols being converted from character image data of at least a portion of the registered document; and a processor configured to extract a symbol set from a plurality of symbols representing a search request as an extracted symbol set, select the extracted symbol set as a search symbol set when the correspondence information indicates that the extracted symbol set satisfies a predetermined condition, and search a document that is assumed to contain the search symbol set using the correspondence information, wherein the predetermined condition is the condition in which a document frequency of the extracted symbol set obtainable from the correspondence information is within a range defined by a first value and a second value, the second value being greater than the first value, and wherein the processor is further configured to generate, from the correspondence information, graphical representation showing a distribution of document frequencies of the registered symbol set, and analyze the distribution of document frequencies of the registered symbol set to generate an analysis result, wherein at least one of the first value and the second value is set based on the analysis result.