diff --git a/src/delta_barth/analysis/forecast.py b/src/delta_barth/analysis/forecast.py
index 26509f7..8d18bc6 100644
--- a/src/delta_barth/analysis/forecast.py
+++ b/src/delta_barth/analysis/forecast.py
@@ -33,7 +33,7 @@ if TYPE_CHECKING:
 
 
 def sales_per_customer(
-    df: pd.DataFrame,
+    data: pd.DataFrame,
     customer_id: int,
     min_num_data_points: int = 100,
 ) -> FcResult:
@@ -53,14 +53,13 @@ def sales_per_customer(
     FcResult
         _description_
     """
-
     cust_data: CustomerDataSalesForecast = CustomerDataSalesForecast()
     # filter data
     # TODO change away from nested DataFrames: just use "f_umsatz_fakt"
     # TODO with strong type checks
-    df = df.copy()
-    df_firma = df[
-        (df["firma_refid"] == customer_id) & (df["beleg_typ"] == 1) & (df["betrag"] > 0)
+    data = data.copy()
+    df_firma = data[
+        (data["firma_refid"] == customer_id) & (data["beleg_typ"] == 1) & (data["betrag"] > 0)
     ]
 
     for transaction in df_firma["vorgang_refid"].unique():