diff --git a/misterdev/core/evolution/partial_credit.py b/misterdev/core/evolution/partial_credit.py
index 472995b..9157e3d 100644
--- a/misterdev/core/evolution/partial_credit.py
+++ b/misterdev/core/evolution/partial_credit.py
@@ -20,6 +20,7 @@
 regressions==0 + resolved_rate (see fitness.py). Pure and deterministic.
 """
 
+import math
 from dataclasses import dataclass
 from typing import Iterable, Optional, Sequence, Tuple
 
@@ -99,8 +100,8 @@ def variance(self) -> float:
         """Sample variance of the per-task credits (population form)."""
         if self.total < 1:
             return 0.0
-        m = self.mean_credit
-        return sum((c - m) ** 2 for c in self.credits) / self.total
+        m = math.fsum(self.credits) / self.total
+        return math.fsum((c - m) ** 2 for c in self.credits) / self.total
 
     @property
     def stderr(self) -> float: