diff --git a/src/facebias/metrics.py b/src/facebias/metrics.py
new file mode 100644
index 0000000..f0b5e9c
--- /dev/null
+++ b/src/facebias/metrics.py
@@ -0,0 +1,51 @@
+# -*- coding: utf-8 -*-
+
+from typing import Any
+
+from facebias.estimators import Capability
+
+
+def find_common_capabilities(
+        gt: dict[str, dict[Capability, Any]],
+        preds: dict[str, dict[Capability, Any]]
+) -> list[str]:
+    """Iterates on `preds` and `gt`, finding common model capabilities.
+
+    Some models predict different features of face images. Some predict sex,
+    age and skin color, while others may predict only one of these features, or
+    others beyond them. This function finds the common capabilities, returning
+    them as a list.
+
+    Parameters
+    ----------
+    gt: dict[str, dict[Capability, Any]]
+        Ground-truth data indexed by element ID, and values are a
+        feature -> prediction dictionary.
+
+    preds: dict[str, dict[Capability, Any]]
+        Predictions data in the same format as `gt`.
+
+    Returns
+    -------
+    common_keys: list[Capability]
+        The common features between `gt` and `preds`. If no common features are
+        found, returns an empty list.
+    """
+    # Find the first common element between the `gt` and `preds`.
+    it = iter(gt)
+    common_elem = ""
+    while True:
+        try:
+            common_elem = next(it)
+        except StopIteration:
+            break
+        else:
+            if common_elem in preds:
+                break
+
+    if not common_elem:
+        return []
+
+    gt_keys = set(gt[common_elem].keys())
+    preds_keys = set(preds[common_elem].keys())
+    return list(gt_keys & preds_keys)