Who stated the PRP? The PRP has been variously attributed (cf. van Rijsbergen, p. 113) to William Maron, William Cooper, and Steve Robertson. Here we use van Rijsbergen’s statement. Dr. Cooper recently offered the following updated opinion concerning the PRP: Date: Wed, 27 Oct 1999 13:36:49-0700 To: rik@cs.ucsd.edu From: Bill <memaron@sims.berkeley.edu> Subject: Information Retrieval Dear Prof. Belew: You did not ask for my thoughts on issues surrounding the so-called “Probability Ranking Principle”, but in the event that you are interested here, very briefly, are three that come to mind immediately. Why rank the output of a retrieval system according to computed values of probability of relevance? Since “probability of relevance” can be interpreted in several different ways, which interpretation is to be preferred? How accurately can those individual probabilities, which are used to compute probability of relevance, be estimated? If we accept (as I do) a frequency interpretation for probability, then it is tautologically true that an event with the higher probability of occurrence will happen (occur) more often than one with a lower value of probability of occurrence. Hence if we are computing probability of relevance for the output in a document retrieval system, the best strategy is to rank those output documents in descending order according to their probabilies of relevance, because by so doing we would be providing the user with an optimal (output) search value. (Assuming here, of course, that all relevant documents are of an equal value.) Looking first at those documents with the highest probability of relevance means that the user will be most successful in finding relevant documents—in the long run. A probability ranking retrieval system is only as good (accurate) as are the estimates of the individual probabilities that are being used to compute the output probability of relevance and upon which the final ranking is based. Which kinds of individual probabilities can be estimated most accurately? How might more accurate estimates be obtained? Since probability of relevance is a relationship among classes of events (individual documents, individual users, documents of a certain type, users of a certain type, etc.), it is important to be clear about we mean, in any given discussion, by “probability of relevance”. In the model proposed in 1960 by Kuhns and myself, probability of relevance is a relationship between a single document and a class of users of a given type (viz., all of those submitting a query of a certain type). In the model proposed in 1976 by Robertson and Sparck-Jones, probability of relevance is a relationship between a given individual and a class of documents of a given type. These are two quite different meanings of “probability of relevance”.