diff --git a/src/calculate_validities.py b/src/calculate_validities.py
index 174964e..ba619d7 100644
--- a/src/calculate_validities.py
+++ b/src/calculate_validities.py
@@ -1,4 +1,5 @@
 from typing import cast
+
 from pandas import DataFrame
 
 
@@ -7,20 +8,22 @@ def calculate_validities(
 ) -> DataFrame:
     EXT_COL_NAME: str = "external_validity"
     INT_COL_NAME: str = "internal_validity"
-    vd = df[(df["design"] == "quasi-experimental") | (df["design"] == "experimental")]
+    cols = {EXT_COL_NAME: 0, INT_COL_NAME: 0}
 
-    vd[EXT_COL_NAME] = 0
-    vd[INT_COL_NAME] = 0
+    vd = df[
+        (df["design"] == "quasi-experimental") | (df["design"] == "experimental")
+    ].copy()
+    vd.assign(**cols)
     vd = cast(DataFrame, vd)
 
     vd[repr_col] = vd[repr_col].fillna("")
+    vd[method_col] = vd[method_col].fillna("")
     # needs to check national before subnational, subnational before local
     vd.loc[vd[repr_col].str.contains("national"), EXT_COL_NAME] = 5.0
     vd.loc[vd[repr_col].str.contains("regional"), EXT_COL_NAME] = 4.0
     vd.loc[vd[repr_col].str.contains("subnational"), EXT_COL_NAME] = 3.0
     vd.loc[vd[repr_col].str.contains("local"), EXT_COL_NAME] = 2.0
 
-    vd[method_col] = vd[method_col].fillna("")
     # needs to go lowest to highest in case of multiple mentioned approaches
     vd.loc[
         vd[method_col].str.contains("|".join(["OLS", "ordinary.least.square"])),
@@ -56,10 +59,12 @@ def calculate_validities(
 
 
 if __name__ == "__main__":
+    import os
     import sys
-    import load_data
-    from pathlib import Path
     from io import StringIO
+    from pathlib import Path
+
+    import load_data
 
     if len(sys.argv) == 2:
         df = load_data.from_yml(Path(sys.argv[1]))
@@ -70,4 +75,8 @@ if __name__ == "__main__":
     output = StringIO()
     df.to_csv(output)
     output.seek(0)
-    print(output.read())
+    try:
+        print(output.read())
+    except BrokenPipeError:
+        devnull = os.open(os.devnull, os.O_WRONLY)
+        os.dup2(devnull, sys.stdout.fileno())