12. In a system for identifying documents matching a query, in which the system has a database containing identifications of documents in a document collection and defining a plurality of representations representing the contents of the documents, the collection compromising a plurality of documents and the database containing a frequency of occurrence of documents containing each of at least some of the representations in the collection of documents, query means for defining a query containing a plurality of concepts, matching means for matching concepts to representations means for selecting documents meeting the query based on frequencies of occurrence of documents in the collection containing representations matching the concepts, and output means responsive to the means for selecting documents for outputting identifications of selected documents, the improvement comprising a process of estimating the frequency of occurrence of documents containing a representation in the collection of documents for which the database does not contain a frequency of occurrence, comprising: identifying, on the basis of concepts in the query, a representation for which the database does not contain a frequency of occurrence; selecting a sample comprising a plurality of documents from the collection, the sample containing fewer documents than the entire collection; identifying the frequency of occurrence of documents containing the identified representation in the selected sample of documents; calculating a maximum and a minimum probable frequency of occurrence of documents containing the identified representation in the collection; and selecting a midpoint of the maximum and minimum probable frequencies as the estimated frequency of occurrence of documents containing the identified representation, whereby the means for selecting documents meeting the query is responsive to the frequencies of occurrence in the database of documents in the collection containing representations matching the concepts and to estimated frequencies of occurrence to select documents in the collection containing representations matching the concepts.
12. In a system for identifying documents matching a query, in which the system has a database containing identifications of documents in a document collection and defining a plurality of representations representing the contents of the documents, the collection compromising a plurality of documents and the database containing a frequency of occurrence of documents containing each of at least some of the representations in the collection of documents, query means for defining a query containing a plurality of concepts, matching means for matching concepts to representations means for selecting documents meeting the query based on frequencies of occurrence of documents in the collection containing representations matching the concepts, and output means responsive to the means for selecting documents for outputting identifications of selected documents, the improvement comprising a process of estimating the frequency of occurrence of documents containing a representation in the collection of documents for which the database does not contain a frequency of occurrence, comprising: identifying, on the basis of concepts in the query, a representation for which the database does not contain a frequency of occurrence; selecting a sample comprising a plurality of documents from the collection, the sample containing fewer documents than the entire collection; identifying the frequency of occurrence of documents containing the identified representation in the selected sample of documents; calculating a maximum and a minimum probable frequency of occurrence of documents containing the identified representation in the collection; and selecting a midpoint of the maximum and minimum probable frequencies as the estimated frequency of occurrence of documents containing the identified representation, whereby the means for selecting documents meeting the query is responsive to the frequencies of occurrence in the database of documents in the collection containing representations matching the concepts and to estimated frequencies of occurrence to select documents in the collection containing representations matching the concepts. 15. The process according to claim 12 where the maximum probable frequency, f.sub.max, and the minimum probable frequency, f.sub.min, are calculated in accordance with the relationships ##EQU6## where n.sub.i is the number of gaps between documents in the sample containing the selected representation, n.sub.c is the number of documents in the collection, x.sub.i is the number of documents in the sample, s.sub.i is the greater of x.sub.i /n.sub.i or the standard deviation of the n.sub.i gaps, and z is the standard critical value for normal distribution for a preselected reliability.