Okapi BM25
BM25
Okapi BM25 - Wikipedia
Practical BM25 - Part 1: How Shards Affect Relevance Scoring in Elasticsearch | Elastic Blog
In Elasticsearch 5.0, we switched to Okapi BM25 as our default similarity algorithm, which is what’s used to score results as they relate to a query.
実践で学ぶBM25 - パート2：BM25のアルゴリズムと変数 | Elastic Blog
http://sonickun.hatenablog.com/entry/2014/11/12/122806