From 41f0b78cbf204649bbae71662a115425c58f048d Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <rhettinger@users.noreply.github.com>
Date: Thu, 14 Mar 2019 02:25:26 -0700
Subject: Simplify overlap() formula for case where variances are equal
 (GH-12323)

---
 Lib/statistics.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Lib/statistics.py b/Lib/statistics.py
index 97f1543..8d79eed 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
-- 
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