FA-74599 / Experiment statistics / Member archive

Bayesian conversion comparison: The sum is normalised by the treatment posterior · case 04

Probabilities can exceed one or fall far below the truth.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

With an integer Beta(prior_a, prior_b) prior, arm posteriors are Beta(prior_a + s, prior_b + n - s). P(B > A) uses the exact integer-parameter sum over i < alpha_B of exp(lnB(alpha_A + i, beta_A + beta_B) - ln(beta_B + i) - lnB(1 + i, beta_B) - lnB(alpha_A, beta_A)). Return [posterior mean A, posterior mean B, P(B > A)] rounded to 6.

Why this case matters

Bayesian dashboards report a "chance to beat control" that product teams act on directly.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
uniform prior small counts	`[0.333333, 0.583333, 1.576417]`	`[0.333333, 0.583333, 0.90081]`	Failed

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