diff --git a/src/calculator.py b/src/calculator.py
index e4e6afd..69ae964 100644
--- a/src/calculator.py
+++ b/src/calculator.py
@@ -1,5 +1,8 @@
 # retoor <retoor@molodetz.nl>
 
+from typing import Sequence
+
+
 def add(left: int, right: int) -> int:
     return left + right
 
@@ -20,3 +23,12 @@ def clamp(value: int, low: int, high: int) -> int:
 
 def clamp_to_byte(value: int) -> int:
     return max(0, min(255, value))
+
+
+def variance(values: Sequence[float]) -> float:
+    if not values:
+        raise ValueError
+    mean = sum(values) / len(values)
+    return sum((x - mean) ** 2 for x in values) / len(values)
+
+
diff --git a/tests/test_calculator.py b/tests/test_calculator.py
index 48d48d4..21de386 100644
--- a/tests/test_calculator.py
+++ b/tests/test_calculator.py
@@ -3,7 +3,7 @@
 import math
 import unittest
 
-from src.calculator import clamp
+from src.calculator import clamp, variance
 
 
 class TestClampFunction(unittest.TestCase):
@@ -61,3 +61,36 @@ class TestClampFunction(unittest.TestCase):
     def test_nan_returns_nan(self) -> None:
         result = clamp(math.nan, 0, 10)
         self.assertTrue(math.isnan(result))
+
+
+class TestVarianceFunction(unittest.TestCase):
+
+    def test_empty_list_raises_value_error(self) -> None:
+        with self.assertRaises(ValueError):
+            variance([])
+
+    def test_empty_tuple_raises_value_error(self) -> None:
+        with self.assertRaises(ValueError):
+            variance(())
+
+    def test_single_element_returns_zero(self) -> None:
+        self.assertEqual(variance([42.0]), 0.0)
+
+    def test_constant_values_return_zero_variance(self) -> None:
+        self.assertEqual(variance([1.0, 1.0, 1.0]), 0.0)
+
+    def test_population_variance_of_known_set(self) -> None:
+        self.assertEqual(variance([1, 2, 3, 4, 5]), 2.0)
+
+    def test_two_element_variance(self) -> None:
+        self.assertEqual(variance([0, 2]), 1.0)
+
+    def test_integer_inputs_return_float(self) -> None:
+        result = variance([10, 20, 30])
+        self.assertIsInstance(result, float)
+        self.assertEqual(result, 200.0 / 3.0)
+
+    def test_tuple_input_returns_variance(self) -> None:
+        self.assertEqual(variance((1, 2, 3, 4, 5)), 2.0)
+
+