IRS13 Word-Word Associations in Document Retrieval Systems chapter M. E. Lesk Harvard University Gerard Salton Use, reproduction, or publication, in whole or in part, is permitted for any purpose of the United States Government. IX-2i is shown at the bottom. It is observed that the associative process removes relevant documents from ranks 10-29, but not 30-99. It also removes documents (but many fewer) from the very low rank positions. The documents in ranks 10-29 which are promoted up to ranks 1-9 are generally those which have had their significant terms upweighted by the process indicated above. The majority of documents promoted from ranks 100-200 had no significant matching terms before associations were added. Of 6 relevant documents which move up over 100 rank positions, all were improved by the recall effect. But in the 10-29 groups, which lost a total of 38 relevant documents, 25 going up and 15 down, nearly every case of improvement is due to the precision effect. This represents a significant improvement in the performance of the relevant documents in this range. In fact, the largest change of ranks in the entire table is the promotion of 16 relevant documents from the 10-19 range to the 1-9 range. In the 100-200 range, a total of 14 documents were promoted out of this range, while 11 were dropped down into it. But it should be noted that if the range 20-99 is considered, as many documents (10) are dropped from these ranges to the 100-200 range as are promoted to them. The net loss of documents from the 100-200 range is due entirely to the 4 promotions to rank positions 1-20, all caused by the recall effect. Another way of describing this effect is to note, for each rank position range, the number of relevant documents promoted as against the number of relevant documents demoted. This is shown in Table 7. Clearly, the system operates well near the 20-29 range; and then again at the very bottom. The last figure in the table is somewhat inflated, since if a docu- ment is already near the bottom, it is difficult to demote if further.