diff --git a/pyproject.toml b/pyproject.toml
index e09b485..46f7532 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "FSRS-Optimizer"
-version = "4.27.4"
+version = "4.27.5"
 readme = "README.md"
 dependencies = [
     "matplotlib>=3.7.0",
diff --git a/src/fsrs_optimizer/fsrs_optimizer.py b/src/fsrs_optimizer/fsrs_optimizer.py
index 6201ec9..97a37ec 100644
--- a/src/fsrs_optimizer/fsrs_optimizer.py
+++ b/src/fsrs_optimizer/fsrs_optimizer.py
@@ -714,7 +714,7 @@ def cal_stability(group: pd.DataFrame) -> pd.DataFrame:
                         power_forgetting_curve,
                         group["delta_t"],
                         group["retention"],
-                        sigma=1 / group["total_cnt"],
+                        sigma=1 / np.sqrt(group["total_cnt"]),
                     )[0][0],
                     1,
                 )