Resolve "add metric MNMB"

Closes #380 (closed)

mlair/helpers/statistics.py

@@ -225,14 +225,28 @@ def index_of_agreement(a, b, dim=None):
     return 1 - frac
 
 
+def modified_normalized_mean_bias(a, b, dim=None):
+    """Calculate modified normalized mean bias (MNMB) where a is the forecast and b the reference (e.g. observation)."""
+    N = np.count_nonzero(a) if len(a.shape) == 1 else a.notnull().sum(dim)
+    return 2 * ((a - b) / (a + b)).sum(dim) / N
+
+
 def calculate_error_metrics(a, b, dim):
-    """Calculate MSE, RMSE, MAE, and IOA. Aditionally, return number of used values for calculation."""
+    """Calculate MSE, RMSE, MAE, IOA, and MNMB. Additionally, return number of used values for calculation.
+
+    :param a: forecast data to calculate metrics for
+    :param b: reference (e.g. observation)
+    :param dim: dimension to calculate metrics along
+
+    :returns: dict with results for all metrics indicated by lowercase metric short name
+    """
     mse = mean_squared_error(a, b, dim)
     rmse = np.sqrt(mse)
     mae = mean_absolute_error(a, b, dim)
     ioa = index_of_agreement(a, b, dim)
+    mnmb = modified_normalized_mean_bias(a, b, dim)
     n = (a - b).notnull().sum(dim)
-    return {"mse": mse, "rmse": rmse, "mae": mae, "ioa": ioa, "n": n}
+    return {"mse": mse, "rmse": rmse, "mae": mae, "ioa": ioa, "mnmb": mnmb, "n": n}
 
 
 def mann_whitney_u_test(data: pd.DataFrame, reference_col_name: str, **kwargs):