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Suppose, instead, that we assume a model
of retrieval costs. Let be the cost of not retrieving
a relevant document and the cost of retrieval of a
nonrelevant document. Then the Probability Ranking
Principle says that
if for a specific document and
for all documents not yet retrieved

(62) 
then is the next document to be retrieved. Such a model gives a formal framework where we can model differential costs of false positives and false negatives and even system performance issues at the modeling stage, rather than simply at the evaluation stage, as we did in Section 8.6 (page ). However, we will not further consider loss/utility models in this chapter.
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