Fix pooled variance calculation

- Fixed calculation of pooled variance for control+test groups.
Original calculation simply averaged variances, without consideration of sample size.

Author: maiyishan

README.md

@@ -1,5 +1,5 @@
-# DABEST-Python
-
+DABEST-Python
+================
 
 <!-- WARNING: THIS FILE WAS AUTOGENERATED! DO NOT EDIT! -->
 

dabest/_stats_tools/confint_2group_diff.py

@@ -314,7 +314,10 @@ def compute_interval_limits(bias, acceleration, n_boots, ci=95):
 
 @njit(cache=True)
 def calculate_group_var(control_var, control_N, test_var, test_N):
-    return control_var / control_N + test_var / test_N
+    
+    pooled_var = ((control_N - 1) * control_var + (test_N - 1) * test_var) / (control_N + test_N - 2) 
+    
+    return pooled_var
 
 
 def calculate_weighted_delta(group_var, differences):

nbs/API/confint_2group_diff.ipynb

@@ -368,7 +368,10 @@
     "\n",
     "@njit(cache=True)\n",
     "def calculate_group_var(control_var, control_N, test_var, test_N):\n",
-    "    return control_var / control_N + test_var / test_N\n",
+    "    \n",
+    "    pooled_var = ((control_N - 1) * control_var + (test_N - 1) * test_var) / (control_N + test_N - 2) \n",
+    "    \n",
+    "    return pooled_var\n",
     "\n",
     "\n",
     "def calculate_weighted_delta(group_var, differences):\n",