Search engines are used to find answers to various types of information problems. Traditionally, search engines are used to identify documents that relate to a query. However, search engines can provide various other types of information. For example, search engines can provide up-to-date weather reports, availability or pricing of airline tickets, current prices for retail goods, or various other type of information. What these types of problems have in common is that the search engine is answering queries that call for objective information that can be marshaled from available data sources.
However, there are some types of queries that are not entirely amenable to objective answers. For example, if a person is asking for a good restaurant in Berkeley, Calif., there may be relevant information that is not objectively discernible from Internet documents. There are professional and amateur restaurant reviews; a search engine can locate these reviews, and the reviews might be of interest to the searcher. However, a searcher who is looking for a restaurant might be interested in more that the opinions of professional reviewers, or the opinions of diners whom the searcher does not know. In general, there is a class of queries that cannot be satisfactorily answered—or that can be only partially answered—by the kind of objective information that that search engines are adept at locating.