From dea74fe5fd06a7778c3f8d1d636f8661de2a372c Mon Sep 17 00:00:00 2001
From: skymanbp <57272723+skymanbp@users.noreply.github.com>
Date: Mon, 24 Aug 2026 03:24:48 -0400
Subject: [PATCH] fix: compute skewness as g1

---
 .../src-internal/DataFrame/Internal/Statistics.hs              | 2 +-
 tests/Operations/Statistics.hs                                 | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/dataframe-operations/src-internal/DataFrame/Internal/Statistics.hs b/dataframe-operations/src-internal/DataFrame/Internal/Statistics.hs
index 0c80ad4a..8f6fcbe3 100644
--- a/dataframe-operations/src-internal/DataFrame/Internal/Statistics.hs
+++ b/dataframe-operations/src-internal/DataFrame/Internal/Statistics.hs
@@ -106,7 +106,7 @@ skewnessStep (SkewAcc !n !meanVal !m2 !m3) !x' =
 computeSkewness :: SkewAcc -> Double
 computeSkewness (SkewAcc n _ m2 m3)
     | n < 3 = 0 -- or error "skewness of <3 samples"
-    | otherwise = (sqrt (fromIntegral n - 1) * m3) / sqrt (m2 ^ (3 :: Int))
+    | otherwise = (sqrt (fromIntegral n) * m3) / sqrt (m2 ^ (3 :: Int))
 {-# INLINE computeSkewness #-}
 
 skewness' :: (VU.Unbox a, Real a, Num a) => VU.Vector a -> Double
diff --git a/tests/Operations/Statistics.hs b/tests/Operations/Statistics.hs
index db907654..1f0d804a 100644
--- a/tests/Operations/Statistics.hs
+++ b/tests/Operations/Statistics.hs
@@ -56,6 +56,7 @@ skewnessOfSymmetricDataSet =
             0
         )
 
+-- g1, matching scipy.stats.skew
 skewnessOfSimpleDataSet :: Test
 skewnessOfSimpleDataSet =
     TestCase
@@ -63,7 +64,7 @@ skewnessOfSimpleDataSet =
             "Skewness of a simple data set"
             ( abs
                 ( D.skewness' (VU.fromList [25 :: Int, 28, 26, 30, 40, 50, 40])
-                    - 0.566_731_633_676
+                    - 0.612_140_127_240_396_6
                 )
                 < 1e-12
             )