PRODUCTION SCENARIO
A retriever is scored on 20 queries, each with exactly one known required passage. The required passage is inside the top five results for 15 queries and inside the top twenty for 18.
What is recall at five for this test set?
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Answer: 75%
Recall is the fraction of relevant documents that are retrieved, so recall at five is the 15 queries whose required passage made the top five divided by the 20 required passages: 75 percent. 90 percent is recall at twenty, computed at the wrong cutoff. 15 percent divides the hits by the 100 result slots the retriever filled, which is precision's denominator. 25 percent is the miss rate reported as if it were recall.