ISR11 Relevance Feedback in an Information Retrieval System chapter W. Riddle T. Horwitz R. Dietz Harvard University Gerard Salton Use, reproduction, or publication, in whole or in part, is permitted for any purpose of the United States Government. VI-li to the user. But this procedure assumes that what is relevant to one user is also relevant to another. Such an assumption is of doubtful value at best. Therefore, there does not appear at present to be any feasible means of returning documents to the user which are actually relevant but [OCRerr][OCRerr]hich, because of the indexing scheme, lie in a region of space far removed from the query and the other relevant documents. Figures 9, 10, and 11 show the effect of using the negative relevance weighting factor heuristic method. Application of the heuristic method to two of the queries show results in a modified query which eventually retrieves the relevant documents. In the third case shown, query OBll, no definite conclusion can be drawn after three modifications have been made; however, it appears that the query vector is being modified correctly, and that it is moving toward the correct region of the document space. The magnitude of the initial value of a affects the speed at which the modified query converges to a position in n-space where it is capable of retrieving all of the relevant documents. For the query shown in Figure 12, when a is initially equal to 1, the final ranhs of the relevant docu- ments are 1, 2, 3, 5, and 19. If, on the other hand, a is initially set equal to 2, the final ranks are 1, 16, 18, 28, and 3[OCRerr]. This dependence on the initial value of a can be explained graphically (see Figure 13). Assume that documents A, B, C, and D are relevant to query q, and that q retrieves documents B, C, and D. Document D has comparitively large concept weights so that when q is modified using an a of 2, the resulting query qt1 is strongly biased toward D. Consequently, q't is unable to retrieve document B. With a milder modification using a smaller value of