21. In a computer 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 comprising a plurality of documents, and query means for defining a query containing a plurality of concepts, apparatus for identifying documents of the document collection containing representations that match the query containing a plurality of concepts, the apparatus comprising: processing means for calculating probabilities that documents match the query and for identifying a first document having a calculated probability; threshold setting means responsive to the processing means for setting a probability threshold equal to the probability of the first document; estimating means responsive to the processing means for estimating a maximum probability for a second document different from the first document based on a partially calculated probability and an assumption that the representations in the second document match the concepts of the query for which probabilities have not been calculated; the processing means being responsive to the estimating means to calculate partial probabilities that representations in the second document match concepts of the query until either the estimated maximum probability for the second document does not at least equal the probability threshold or the probability is calculated for all the concepts in the query; the estimating means being further responsive to the processing means ceasing or completing the calculation of the probability for the second document to estimate a maximum probability for a third document different from the first and second documents; and output means responsive to the processing means for outputting identifications of only documents whose probability is calculated for all concepts in the query.
23. Apparatus according to claim 21 wherein the threshold setting means is responsive to the processing means calculating the probability for the second document for all the concepts in the query to set the probability threshold equal to the probability of the second document.