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-21 the initial result. The co-occurrence correlation function ;[OCRerr]th the constant alpha strategy achieves the greatest increase in the first and second updates, an4 the same correlation function, but with increasing alpha strategy, gives the best result for the third update. Co[OCRerr][OCRerr]binations using the co-occurrence and cosine correlation functions together with the constant and increasing alpha strategies perform the best. Combinations using the simple vector matching correlation function and the alpha correlations strategy always have low merit. Another method of displaying the increase in retrieval performance achieved by relevance feedback is the use of a plot of precision versus recall. Table 1312 gives such a plot of the same results as those given in Table 131, in which the cosine correlation function and increasing alpha strategy is used. The results of the 22 requests are averaged by the use of a cut-off made after examination of m consecutive documents (m ranging from 1 to 20), and the precision and recall values are averaged over all requests at each cut-off point t- obtain the nine average point for each curve. The four performance curves how the initial search and the three updated searches The first update gives the biggest improvement in performance, and the updated searches achieve a very substantial improvement in performanc[OCRerr] at the high precision end of the curves This improvement decreases as recall increases, and [OCRerr]s almost lost when 0.7 recall is reached. If performance is compared at a cut-off of four documents, for example, the third updated search achieves an improvement over the initial search of more than 0.3 in both precision and recall. To summarize, evaluation of the relevance feedback methods averaged over 22 search requests shows a very considerable improvement in performance