BM25 and nearest neighbor have two different score scales. Reciprocal rank fusion allows both lexical and symantic search result scores to be mixed into one stack ranked list.
(This note is under construction)
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Rather than score search results on the probability that the query is relevant to a document, BM25 provides a ranking of probability. That’s because the probability the query appears in the document doesn’t actually matter to the results. This is a heuristic that makes the algorithm efficient and provide excellent results.