Exercise 22.11

Consider the problem of trying to evaluate the quality of an IR system that returns a ranked list of answers (like most Web search engines). The appropriate measure of quality depends on the presumed model of what the searcher is trying to achieve, and what strategy she employs. For each of the following models, propose a corresponding numeric measure.

1. The searcher will look at the first twenty answers returned, with the objective of getting as much relevant information as possible.

2. The searcher needs only one relevant document, and will go down the list until she finds the first one.

3. The searcher has a fairly narrow query and is able to examine all the answers retrieved. She wants to be sure that she has seen everything in the document collection that is relevant to her query. (E.g., a lawyer wants to be sure that she has found all relevant precedents, and is willing to spend considerable resources on that.)

4. The searcher needs just one document relevant to the query, and can afford to pay a research assistant for an hour’s work looking through the results. The assistant can look through 100 retrieved documents in an hour. The assistant will charge the searcher for the full hour regardless of whether he finds it immediately or at the end of the hour.

5. The searcher will look through all the answers. Examining a document has cost \$A; finding a relevant document has value \$ B; failing to find a relevant document has cost \\$ C for each relevant document not found.

6. The searcher wants to collect as many relevant documents as possible, but needs steady encouragement. She looks through the documents in order. If the documents she has looked at so far are mostly good, she will continue; otherwise, she will stop.

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