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Retrieval & EmbeddingsHard· BM25· IDF· Information Retrieval
Where does BM25’s IDF term come from, and why that form...
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See plansA strong answer follows a four-step chain: rank by probability of relevance, convert to odds and drop the query-constant prior, factorize under binary independence, and estimate from a 2x2 table. By setting the relevance parameter S to 0, the formula ln(N-nt+0.5 / nt+0.5) is derived. A weak answer merely describes the behavior of downweighting common words without explaining the mathematical origin.