This paper describes an all computer document retrieval system which can find documents related to a request even though they may not be indexed by the exact terms of the request, and can present these documents in the order of their relevance to the request.The key to this ability lies in the application of a statistical formula by which the computer calculates the degree of association between pairs of index terms.With proper manipulation of these associations (entirely within the machine) a vocabulary of synonyms, near synonyms and other words closely related to any given term or group of terms is derived.Such a vocabulary related to a group of request terms is believed to be a much more powerful tool for selecting documents from a collection than has been available heretofore.By noting the number of matching terms between this extended list of request terms and the terms used to index a document, and with due regard for their degree of association, documents are selected by the computer and arranged in the order of their relevance to the request.