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. VII-43 2. For a high precision need, only IRE-3 produces some advantage to phrases (Figs. 21 and 25); however, this is based on a small superiority for one or two requests only, and is not considered significant (Figs. 26 and 27). 3. For a high recall need, use of the average rank of the last relevant shows the phrases to be useful on IRE-3 only (Fig. 25) by a small margin on an individual request basis (Fig. 26). Results comparing the thesaurus with the addition of various hierarchy relations on the IRE-3 collection produce the following con- clus ions: 1. Thesaurus alone is always superior to hierarchy on three of the relations tested, and on two others ("parents and "all" relations), the hierarchy gives a small advantage over portions of the precision recall curve (Figs. 22, 23). On an individual request basis (Fig 24), the thesaurus is equal to "parents", and superior to "all" relations; the hierarchy is thus not to be preferred. 2. For a high precision need, Fig. 25 suggests that some advantage accrues, but Fig. 27 shows that its success is limited to one or two requests that do badly with the thes- aurus alone. 3. For a high recall need, Fig. 25 shows that the hierarchy performs well, but Fig. 26 reveals again that it achieves only a few dramatically good results with a poorer average high recall per[OCRerr]ormance for individual requests than thesaurus. 7. Performance Analyses The first task of the analysis is to explain the mechanism which causes an improvement in retrieval performance using the thesaurus and