Equation 2 · What Actually Happens Between a Query and an Answer in RAG
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Symbol b
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What the article says around this equation
Robertson and Zaragoza’s account of the probabilistic relevance framework behind this formula is worth reading past the equation for one design choice it exposes: the term-frequency component saturates rather than growing linearly, so a document repeating a query term fifty times is scored only marginally higher than one repeating it five times, and the length-normalisation term b trades off penalising long documents against rewarding genuinely comprehensive ones [ 1 ] . BM25 has no notion of meaning. A query for “cannot authenticate” and a document about “login failure” share no terms and score zero, regardless of how obviously related a person would find them.
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