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)
+
+