IRS13 Thesaurus, Phrase and Hierarchy Dictionaries chapter E. M. Keen Harvard University Gerard Salton Use, reproduction, or publication, in whole or in part, is permitted for any purpose of the United States Government. Vil-si Examples of the improvement given to two of the non-staff requests by the hierarchy are given in Fig. 31. Request Q006 asks for documents about information retrieval using computers, and concept 26 "retrieval" is linked in the hierarchy to "parent" concept 200 "data-processing", "data handling", etc. All six relevant documents also contain concept 200 as a result of the hierarchy expansion; one document did not originally contain concept 26, and so obtained concept 200 from "sums" other than concept 26; the other documents achieved high weights on concept 200 through a similar connection. Thus, concept 200 is in the main respon- sible for the sharp improvement in performance, mainly through the mechanism of increasing the weight given to the notion vital to the request. Request Q015 has six concepts in the request when the thesaurus is used, and this is expanded to twenty-six when the hierarchy "all" relation is in use. Document 200 has a greatly improved rank on hierarchy, because all but two of the additional request concepts added by hierarchy are matched, thus giving a total match of 23 out of 26 on hierarchy, although 5 out of 6 matches were achieved by thesaurus. In general, it is clearly unusual for a document to match with nearly all the hierarchy expansions in a given request, and the case of document 200 may be a special one. Documen[OCRerr]l06 and 382 both exhibit cases of hierarchy acting as a recall device, since request concepts 383 ("Transcendental") and 618 ("Function") do not match with the thesaurus, but do match with hierarchy through "brothers" and "cross reference" relations. This points to the probably reason why the hierarchy as tested is not generally effective: because the use of thesaurus groups to build