From: Raymond Hettinger <rhettinger@users.noreply.github.com>
Date: Thu, 14 Mar 2019 09:25:26 +0000 (-0700)
Subject: Simplify overlap() formula for case where variances are equal (GH-12323)
X-Git-Tag: v3.8.0a3~104
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=41f0b78cbf204649bbae71662a115425c58f048d;p=thirdparty%2FPython%2Fcpython.git

Simplify overlap() formula for case where variances are equal (GH-12323)
---

diff --git a/Lib/statistics.py b/Lib/statistics.py
index 97f154373dc0..8d79eed6b1aa 100644
--- a/Lib/statistics.py
+++ b/Lib/statistics.py
@@ -773,7 +773,7 @@ class NormalDist:
         dv = Y_var - X_var
         dm = fabs(Y.mu - X.mu)
         if not dv:
-            return 2.0 * NormalDist(dm, 2.0 * X.sigma).cdf(0)
+            return 1.0 - erf(dm / (2.0 * X.sigma * sqrt(2.0)))
         a = X.mu * Y_var - Y.mu * X_var
         b = X.sigma * Y.sigma * sqrt(dm**2.0 + dv * log(Y_var / X_var))
         x1 = (a + b) / dv