diff --git a/pcntoolkit/dataio/norm_data.py b/pcntoolkit/dataio/norm_data.py
index 994b5655..fbc07f14 100644
--- a/pcntoolkit/dataio/norm_data.py
+++ b/pcntoolkit/dataio/norm_data.py
@@ -11,7 +11,7 @@
 from __future__ import annotations
 
 import copy
-import json
+from filelock import FileLock
 import os
 from collections import defaultdict
 from functools import reduce
diff --git a/pcntoolkit/normative_model.py b/pcntoolkit/normative_model.py
index 05f16559..1d87b26f 100644
--- a/pcntoolkit/normative_model.py
+++ b/pcntoolkit/normative_model.py
@@ -9,6 +9,7 @@
 import importlib.metadata
 import json
 import os
+import warnings
 from typing import List, Optional, Tuple, Union
 
 import numpy as np
@@ -205,6 +206,14 @@ def transfer(self, transfer_data: NormData, save_dir: str | None = None, **kwarg
         new_model.response_vars = respvar_intersection
 
         new_model.preprocess(transfer_data)
+
+if hasattr(self, "batch_effects") and hasattr(transfer_data, "batch_effects"):
+    if len(transfer_data.batch_effects) < len(self.batch_effects):
+        warnings.warn(
+            "Transfer dataset contains fewer batch effects than the training dataset. "
+            "This may lead to biased transfer corrections.",
+            UserWarning,
+        )
         new_model.register_batch_effects(transfer_data)
 
         Output.print(Messages.TRANSFERRING_MODELS, n_models=len(respvar_intersection))