diff --git a/grouped_run_statistics/src/grouped_run_statistics/core.py b/grouped_run_statistics/src/grouped_run_statistics/core.py
new file mode 100644
index 0000000..db33611
--- /dev/null
+++ b/grouped_run_statistics/src/grouped_run_statistics/core.py
@@ -0,0 +1,94 @@
+from __future__ import annotations
+
+import logging
+from collections import Counter
+from typing import List, Dict, Any
+
+__all__ = ["group_runs"]
+
+
+logger = logging.getLogger(__name__)
+if not logger.handlers:
+    handler = logging.StreamHandler()
+    formatter = logging.Formatter("%(asctime)s - %(levelname)s - %(message)s")
+    handler.setFormatter(formatter)
+    logger.addHandler(handler)
+    logger.setLevel(logging.INFO)
+
+
+class InvalidRunRecordError(ValueError):
+    """Custom exception raised when run records are invalid."""
+
+
+class GroupedStatistics(Dict[str, int]):
+    """Represents aggregated statistics grouped by pinned/unpinned and result status."""
+
+
+REQUIRED_FIELDS = {"id", "pinned", "status"}
+VALID_STATUSES = {"PASS", "WARN", "FAIL"}
+
+
+def group_runs(run_data: List[Dict[str, Any]]) -> Dict[str, int]:
+    """
+    Gruppiert Testlaufdaten in 'pinned' und 'unpinned' Gruppen und zählt PASS/WARN/FAIL.
+
+    Args:
+        run_data: Liste von Dictionaries mit Schlüsseln 'id', 'pinned' (bool), 'status' (str).
+
+    Returns:
+        Dict[str, int]: Aggregierte Statistik der Runs nach pinned/unpinned und Status.
+    """
+
+    # Input validation
+    if not isinstance(run_data, list):
+        raise InvalidRunRecordError("Input run_data must be a list of dictionaries.")
+
+    stats_counter = Counter(
+        {
+            "pinned_pass": 0,
+            "unpinned_pass": 0,
+            "pinned_warn": 0,
+            "unpinned_warn": 0,
+            "pinned_fail": 0,
+            "unpinned_fail": 0,
+        }
+    )
+
+    for idx, record in enumerate(run_data):
+        if not isinstance(record, dict):
+            raise InvalidRunRecordError(f"Record at index {idx} is not a dictionary.")
+
+        if not REQUIRED_FIELDS.issubset(record.keys()):
+            raise InvalidRunRecordError(
+                f"Record at index {idx} missing required fields: {REQUIRED_FIELDS - record.keys()}"
+            )
+
+        pinned = record["pinned"]
+        status = record["status"].upper()
+
+        if not isinstance(pinned, bool):
+            raise InvalidRunRecordError(f"Record at index {idx} has non-boolean 'pinned' field.")
+
+        if status not in VALID_STATUSES:
+            raise InvalidRunRecordError(
+                f"Record at index {idx} has invalid status '{status}'. Must be one of {VALID_STATUSES}."
+            )
+
+        key_prefix = "pinned" if pinned else "unpinned"
+        key = f"{key_prefix}_{status.lower()}"
+        stats_counter[key] += 1
+
+    result: GroupedStatistics = GroupedStatistics(stats_counter)
+
+    logger.info(
+        "Grouped %d runs into statistics: pinned_pass=%d, unpinned_pass=%d, pinned_warn=%d, unpinned_warn=%d, pinned_fail=%d, unpinned_fail=%d",
+        len(run_data),
+        result["pinned_pass"],
+        result["unpinned_pass"],
+        result["pinned_warn"],
+        result["unpinned_warn"],
+        result["pinned_fail"],
+        result["unpinned_fail"],
+    )
+
+    return dict(result)