diff --git a/mlair/helpers/statistics.py b/mlair/helpers/statistics.py
index 15cd926b0f84ea361e5e4fef0622bab0be9fe910..c897f875c56da820512c0de58aca34f9cf0fc4cb 100644
--- a/mlair/helpers/statistics.py
+++ b/mlair/helpers/statistics.py
@@ -646,7 +646,7 @@ def calculate_bias_free_data(data, time_dim="index", window_size=30):
     bias_free_data = data - overall_mean
 
     # seasonal mean
-    seasonal_mean = data.rolling(**{time_dim: window_size}, min_periods=int(window_size/2)).mean()
+    seasonal_mean = data.rolling(**{time_dim: window_size}, min_periods=int(window_size/2), center=True).mean()
     seasonal_bias_free_data = data - seasonal_mean
 
     return bias_free_data, seasonal_bias_free_data
\ No newline at end of file