ISR10 Evaluation of Document Retrieval Systems chapter Joseph John Rocchio Harvard University Gerard Salton Use, reproduction, or publication, in whole or in part, is permitted for any purpose of the United States Government. 5-50 retrieved sequence, exibits a reasonable range for the search requests examined to date, and typically varies from .6 to 1.0. In practice then, to produce a useful range of values for the recall index, one is forced to expand its scale A scale expansion of 5, introduced so as to main-. tam an upper bound of 1.0, produces an observed range for the scaled recall h.ndex similar to that of the precision index. The scaled index is defined as: rn = 1.0 - 5(i.o[OCRerr]- rn) 5 where r is the normalized rank recall defined by equation (5.20). n Two related performance indices may be derived from the two which have been considered. These are useful in the case where a par- ticular query is subjected to a set of retrieval operations (varying some system parameter for example) which are to be compared. The recall error, equation (5.18) was derived as: n +1 recall error = - 0 2 Since 0 = (n+1) / 2, a positive index with an upper bound of 1.0 may mm .0 be defined as: n +1 .0 2 rank recall = A similar observation for the case of the derived precision error, equation (5.21), produces the index: