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 ~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