8. A system for identifying documents matching a comprising: a memory containing a database containing identification of documents in a document collection and defining a plurality of representations representing the contents of the documents, the collection comprising a plurality of documents, the database further containing indications of the frequencies of occurrence of documents containing first representations in the collection; computer means responsive to a query defining a plurality of concepts, the computer means including matching means for matching the concepts to representations, estimating means for estimating the frequency of occurrence of documents containing a second selected representation in the collection, the second selected representation being different from any of the first representations, the estimating means including sample selection means for selecting a sample comprising a plurality of documents from the collection, the sample containing fewer documents than the entire collection; frequency identifying means responsive to the sample selection means for identifying the frequency of occurrence of documents containing the second selected representation in the selected sample of documents; processor means responsive to the memory means and to the frequency identifying means for calculating a maximum and a minimum probable frequency of occurrence of documents containing the second selected representation in the collection; and selection means responsive to the processor means for selecting the midpoint, of the maximum and minimum probable frequencies as the estimated frequency of occurrence of the second selected representation; retrieval means for selecting documents meeting the query based on the frequencies of occurrence of documents containing first representations which match the concepts and the estimated frequencies of occurrence of documents containing second representations which match the concepts, and output means responsive to the retrieval means and the memory for outputting identifications of the selected documents.
8. A system for identifying documents matching a comprising: a memory containing a database containing identification of documents in a document collection and defining a plurality of representations representing the contents of the documents, the collection comprising a plurality of documents, the database further containing indications of the frequencies of occurrence of documents containing first representations in the collection; computer means responsive to a query defining a plurality of concepts, the computer means including matching means for matching the concepts to representations, estimating means for estimating the frequency of occurrence of documents containing a second selected representation in the collection, the second selected representation being different from any of the first representations, the estimating means including sample selection means for selecting a sample comprising a plurality of documents from the collection, the sample containing fewer documents than the entire collection; frequency identifying means responsive to the sample selection means for identifying the frequency of occurrence of documents containing the second selected representation in the selected sample of documents; processor means responsive to the memory means and to the frequency identifying means for calculating a maximum and a minimum probable frequency of occurrence of documents containing the second selected representation in the collection; and selection means responsive to the processor means for selecting the midpoint, of the maximum and minimum probable frequencies as the estimated frequency of occurrence of the second selected representation; retrieval means for selecting documents meeting the query based on the frequencies of occurrence of documents containing first representations which match the concepts and the estimated frequencies of occurrence of documents containing second representations which match the concepts, and output means responsive to the retrieval means and the memory for outputting identifications of the selected documents. 10. The system according to claim 8 where the processor means calculates the maximum probable frequency, f.sub.max, and the minimum probable frequency, f.sub.min, in accordance with relationships based on the number of gaps between documents in the sample containing the second selected representation (n.sub.i), the number of documents in the collection (n.sub.c), and the number of documents in the sample (x.sub.i).