Keyword Search 101: How BM25 Scores

Deployed's explainer works through BM25, the default ranker in Lucene, Elasticsearch, OpenSearch and Solr, on a five-page example and then on 660 questions over papers, contracts and meeting transcripts. It reports that IDF and term saturation matter most, that the defaults k1 = 1.2 and b = 0.75 land within 1.5 points of the best of 70 settings, and that scores shouldn't be thresholded. Textbook IDF can go negative, so Lucene's ln(1 + ...) is safer. The measurements are the author's own, on small corpora, with no source given.