Editing Full-text search (section)

==The precision vs. recall tradeoff==
[[Image:Full-text-search-results.png|150px|thumb|right|Diagram of a low-precision, low-recall search]]
Recall measures the quantity of relevant results returned by a search, while precision is the measure of the quality of the results returned. Recall is the ratio of relevant results returned to all relevant results. Precision is the ratio of the number of relevant results returned to the total number of results returned.

The diagram at right represents a low-precision, low-recall search. In the diagram the red and green dots represent the total population of potential search results for a given search. Red dots represent irrelevant results, and green dots represent relevant results. Relevancy is indicated by the proximity of search results to the center of the inner circle. Of all possible results shown, those that were actually returned by the search are shown on a light-blue background. In the example only 1 relevant result of 3 possible relevant results was returned, so the recall is a very low ratio of 1/3, or 33%. The precision for the example is a very low 1/4, or 25%, since only 1 of the 4 results returned was relevant.<ref name="isbn1430215941">{{cite book|last=Coles|first=Michael|year=2008|title=Pro Full-Text Search in SQL Server 2008|edition=Version 1|publisher=[[Apress|Apress Publishing Company]]|isbn=978-1-4302-1594-3}}</ref>

Due to the ambiguities of [[natural language]], full-text-search systems typically includes options like [[stop word|filtering]] to increase precision and [[stemming]] to increase recall. [[Controlled vocabulary|Controlled-vocabulary]] searching also helps alleviate low-precision issues by [[tag (metadata)|tagging]] documents in such a way that ambiguities are eliminated. The trade-off between precision and recall is simple: an increase in precision can lower overall recall, while an increase in recall lowers precision.<ref name="YuwonoLee">{{Cite conference | first = Yuwono | last = B. |author2=Lee, D. L. | title = Search and ranking algorithms for locating resources on the World Wide Web | pages = 164 | publisher = 12th International Conference on Data Engineering (ICDE'96) | year = 1996}}</ref>

{{See also|Precision and recall}}