IRS13 Search Matching Functions 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. 111-17 Comparisons of individual request merit are given in Figure 8, where 58.8% to 76.5% of the requests favor cosine on IRE-3, 70.0% to 85.7% favor cosine on Cran-l, and 50% to 80% favor cosine on ADI. The use of the thesaurus dictionary, which gives a performance superior to stem on all collections, also shows good superiority for cosine. C) Analysis of Performance Since cosine consistently performs better than overlap, an adequate explanation must be sought. It should be noted that although the average results show cosine to be superior by only around 5% in precision, individual requests and relevant documents may show large changes in favor of both cosine and overlap. Figures 9 and 10 show results which strongly favor both cosine and overlap, respectively, for two individual requests. The individual relevant documents display large changes in rank with change in correlation coefficient. Using the Cran-l Stem results, it is found that of 198 documents relevant to all 42 requests, 95 show rank improvements on cosine over over- lap, 62 show the reverse improvement, and [OCRerr]l show no change in rank. Figure 11 shows the amounts of change in rank for the 95 and 62 documents, revealing that the advantage is, as expected, with cosine. Figure 12 gives a diagrammatic representation of what is happening when the ranking induced by overlap is changed to cosine. Overlap orders documents by match alone; to simplify the diagram five matching strengths only are recorded. At each matching strength both relevant and non-relevant documents may be found intermingled. If the ordering induced by cosine is now imposed on the documents, two types of changes take place. Ffrstly, some non-relevant documents are decreased in match and rank position, and some relevant documents are increased in match and rank position; naturally, such changes favor cosine as opposed to overlap. Secondly, the changes that