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Query expansion

An example of query expansion in the interface of the Yahoo! web search engine in 2006.The expanded query suggestions appear just below the ``Search Results'' bar.

In relevance feedback, users give additional input on documents (by marking documents in the results set as relevant or not), and this input is used to reweight the terms in the query for documents. In query expansion on the other hand, users give additional input on query words or phrases, possibly suggesting additional query terms. Some search engines (especially on the web) suggest related queries in response to a query; the users then opt to use one of these alternative query suggestions. Figure 9.6 shows an example of query suggestion options being presented in the Yahoo! web search engine. The central question in this form of query expansion is how to generate alternative or expanded queries for the user. The most common form of query expansion is global analysis, using some form of thesaurus. For each term $t$ in a query, the query can be automatically expanded with synonyms and related words of $t$ from the thesaurus. Use of a thesaurus can be combined with ideas of term weighting: for instance, one might weight added terms less than original query terms.

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% latex2html id marker 11688
\begin{itemize}
\item User query: \t...
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Medline vocabulary as shown.}\end{figure}

Methods for building a thesaurus for query expansion include:

Thesaurus-based query expansion has the advantage of not requiring any user input. Use of query expansion generally increases recall and is widely used in many science and engineering fields. As well as such global analysis techniques, it is also possible to do query expansion by local analysis, for instance, by analyzing the documents in the result set. User input is now usually required, but a distinction remains as to whether the user is giving feedback on documents or on query terms.


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2009-04-07