# Why isn't cosine similarity implemented?

**URL:** <https://discuss.elastic.co/t/why-isnt-cosine-similarity-implemented/172446>\
**Category:** Elasticsearch\
**Created:** [March 15, 2019, 1:03am UTC](https://discuss.elastic.co/t/why-isnt-cosine-similarity-implemented/172446 "2019-03-15T01:03:46Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![danielspeixoto](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/danielspeixoto/32/42052_2.png) [@danielspeixoto](https://discuss.elastic.co/u/danielspeixoto)\
**Post date:** [March 15, 2019, 1:03am UTC](https://discuss.elastic.co/t/why-isnt-cosine-similarity-implemented/172446/1 "2019-03-15T01:03:46Z")

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I'm conducting a research using elasticsearch. I was planning to use cosine similarity but I noted that it is unavailable and instead we have BM25 as default scoring function.  
Is there a reason for that? Is cosine similarity improper for querying documents? Why was BM25 chosen as default?  
Thanks

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**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [April 12, 2019, 1:03am UTC](https://discuss.elastic.co/t/why-isnt-cosine-similarity-implemented/172446/2 "2019-04-12T01:03:49Z")

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Closed on April 12, 2019, 1:03am UTC

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