fix: adjust duckdb population skewness by correction factor to get sa…

…mple skewness (#2021)
narwhals-dev · Feb 16, 2025 · 3aefc49 · 3aefc49
1 parent 6d07f89
commit 3aefc49
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Showing 2 changed files with 4 additions and 3 deletions.
diff --git a/narwhals/_duckdb/expr.py b/narwhals/_duckdb/expr.py
@@ -317,7 +317,10 @@ def func(_input: duckdb.Expression) -> duckdb.Expression:
                     condition=(count == lit(1)), value=lit(float("nan"))
                 ).otherwise(
                     CaseExpression(condition=(count == lit(2)), value=lit(0.0)).otherwise(
+                        # Adjust population skewness by correction factor to get sample skewness
                         FunctionExpression("skewness", _input)
+                        * (count - lit(2))
+                        / FunctionExpression("sqrt", count * (count - lit(1)))
                     )
                 )
             )

diff --git a/tests/expr_and_series/unary_test.py b/tests/expr_and_series/unary_test.py
@@ -10,9 +10,7 @@
 from tests.utils import assert_equal_data
 
 
-def test_unary(constructor: Constructor, request: pytest.FixtureRequest) -> None:
-    if "duckdb" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_unary(constructor: Constructor) -> None:
     data = {
         "a": [1, 3, 2],
         "b": [4, 4, 6],