ISR11 Design Criteria for Automatic Information Systems chapter M. E. Lesk G. Salton Harvard University Gerard Salton Use, reproduction, or publication, in whole or in part, is permitted for any purpose of the United States Government. v-28 E) Hierarchical Subject Expansion Hierarchical arrangements of information identifiers, similar in construction to library classification schedules make it possible, given an entry, to find more general terms by going `up' in the hierarchy (expansion by parents), and more specific ones by going "down" (expansion by sons). The hierarchies provided for the SM[OCRerr][OCRerr]T system include, in addition, expansions by "brothers" on the same level as the original terms, and expansions by adding certain "cross-references". Dozens of different hierarchy options can be used, of which two are shown in Fig. 13. Fig. 13(a) shows an expansion by adding for each original term its parent in the hierarchy, the expansion being applied to both documents and requests. Clearly, this option does not on the average provide an improvement over the standard t'Harris Three" thesaurus process. On the other hand, an expansion by "sons1t applied to requests only (and not to the documents) seems to offer some improvement in performance for the middle ranges of recall and precision. In general, hierarchical subject expansions result in large-scale disturbances in the information identifiers attached to documents and search requests. Occasionally, such a disturbance can serve to crystallize the meaning of a poorly stated request, particularly if the request is far removed from the principal subjects covered by the document collection. More often, the change in direction specified by the hierarchy option is too violent, and the average performance of most hierarchy procedures does not appear to be sufficiently promising to advocate their [OCRerr]ncorporation in an analysis system for automatic document retrieval.