BM25, dense retrieval, and cross-encoder reranking

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Concept Dense vector retrieval (Module 3's prior two lessons) is excellent at matching semantic intent, but it's not the only signal worth using — and in production RAG pipelines, the strongest systems combine multiple retrieval signals rather than relying on embeddings alone. BM25 is a decades-old sparse keyword-matching algorithm (the modern successor to TF-IDF) that scores documents by exact term……

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