Most popular web search engines are optimized for precision—getting that useful document in the in the top five or ten hits so that the user doesn’t have to page through the results to find it. This works well for known-item search (finding an address of a restaurant, a birthday of a movie star, etc.) and for searches that rely on combinations of keywords.

But some kinds of information needs don’t fit that pattern well. Sometimes the information being sought is spread over multiple documents, sometimes people need to find multiple instances of documents that match some query to compare or contrast them, etc. The task becomes more recall- rather than precision-oriented. Furthermore, these searches may be repeated over time, as the user finds information that causes the information need to change. Medical information seeking is one obvious such example. Are there others?

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