diff --git a/tracetools_analysis/tracetools_analysis/data_model/__init__.py b/tracetools_analysis/tracetools_analysis/data_model/__init__.py
index 4451949..4459da6 100644
--- a/tracetools_analysis/tracetools_analysis/data_model/__init__.py
+++ b/tracetools_analysis/tracetools_analysis/data_model/__init__.py
@@ -15,6 +15,13 @@
 
 """Base data model module."""
 
+from typing import Any
+from typing import Dict
+from typing import List
+
+
+DataModelIntermediateStorage = List[Dict[str, Any]]
+
 
 class DataModel():
     """
diff --git a/tracetools_analysis/tracetools_analysis/data_model/cpu_time.py b/tracetools_analysis/tracetools_analysis/data_model/cpu_time.py
index b66c7e4..f255e16 100644
--- a/tracetools_analysis/tracetools_analysis/data_model/cpu_time.py
+++ b/tracetools_analysis/tracetools_analysis/data_model/cpu_time.py
@@ -18,6 +18,7 @@
 import pandas as pd
 
 from . import DataModel
+from . import DataModelIntermediateStorage
 
 
 class CpuTimeDataModel(DataModel):
@@ -30,10 +31,7 @@ class CpuTimeDataModel(DataModel):
     def __init__(self) -> None:
         """Create a CpuTimeDataModel."""
         super().__init__()
-        # Intermediate
-        self._times = []
-        # Final
-        self.times = None
+        self._times: DataModelIntermediateStorage = []
 
     def add_duration(
         self,
diff --git a/tracetools_analysis/tracetools_analysis/data_model/memory_usage.py b/tracetools_analysis/tracetools_analysis/data_model/memory_usage.py
index 7785cbc..0eabf8a 100644
--- a/tracetools_analysis/tracetools_analysis/data_model/memory_usage.py
+++ b/tracetools_analysis/tracetools_analysis/data_model/memory_usage.py
@@ -18,6 +18,7 @@
 import pandas as pd
 
 from . import DataModel
+from . import DataModelIntermediateStorage
 
 
 class MemoryUsageDataModel(DataModel):
@@ -31,10 +32,7 @@ class MemoryUsageDataModel(DataModel):
     def __init__(self) -> None:
         """Create a MemoryUsageDataModel."""
         super().__init__()
-        # Intermediate
-        self._memory_diff = []
-        # Final
-        self.memory_diff = None
+        self._memory_diff: DataModelIntermediateStorage = []
 
     def add_memory_difference(
         self,
diff --git a/tracetools_analysis/tracetools_analysis/data_model/profile.py b/tracetools_analysis/tracetools_analysis/data_model/profile.py
index 6d1bcd9..0845647 100644
--- a/tracetools_analysis/tracetools_analysis/data_model/profile.py
+++ b/tracetools_analysis/tracetools_analysis/data_model/profile.py
@@ -20,6 +20,7 @@ from typing import Optional
 import pandas as pd
 
 from . import DataModel
+from . import DataModelIntermediateStorage
 
 
 class ProfileDataModel(DataModel):
@@ -33,10 +34,7 @@ class ProfileDataModel(DataModel):
     def __init__(self) -> None:
         """Create a ProfileDataModel."""
         super().__init__()
-        # Intermediate
-        self._times = []
-        # Final
-        self.times = None
+        self._times: DataModelIntermediateStorage = []
 
     def add_duration(
         self,
diff --git a/tracetools_analysis/tracetools_analysis/data_model/ros2.py b/tracetools_analysis/tracetools_analysis/data_model/ros2.py
index 8c3a019..7c0799c 100644
--- a/tracetools_analysis/tracetools_analysis/data_model/ros2.py
+++ b/tracetools_analysis/tracetools_analysis/data_model/ros2.py
@@ -18,6 +18,7 @@
 import pandas as pd
 
 from . import DataModel
+from . import DataModelIntermediateStorage
 
 
 class Ros2DataModel(DataModel):
@@ -30,36 +31,19 @@ class Ros2DataModel(DataModel):
     def __init__(self) -> None:
         """Create a Ros2DataModel."""
         super().__init__()
-
-        # Intermediate
-        self._contexts = []
-        self._nodes = []
-        self._publishers = []
-        self._subscriptions = []
-        self._subscription_objects = []
-        self._services = []
-        self._clients = []
-        self._timers = []
-        self._callback_objects = []
-        self._callback_symbols = []
-        self._callback_instances = []
-
-        # Final
         # Objects (one-time events, usually when something is created)
-        self.contexts = None
-        self.nodes = None
-        self.publishers = None
-        self.subscriptions = None
-        self.subscription_objects = None
-        self.services = None
-        self.clients = None
-        self.timers = None
-
-        self.callback_objects = None
-        self.callback_symbols = None
-
+        self._contexts: DataModelIntermediateStorage = []
+        self._nodes: DataModelIntermediateStorage = []
+        self._publishers: DataModelIntermediateStorage = []
+        self._subscriptions: DataModelIntermediateStorage = []
+        self._subscription_objects: DataModelIntermediateStorage = []
+        self._services: DataModelIntermediateStorage = []
+        self._clients: DataModelIntermediateStorage = []
+        self._timers: DataModelIntermediateStorage = []
+        self._callback_objects: DataModelIntermediateStorage = []
+        self._callback_symbols: DataModelIntermediateStorage = []
         # Events (multiple instances, may not have a meaningful index)
-        self.callback_instances = None
+        self._callback_instances: DataModelIntermediateStorage = []
 
     def add_context(
         self, context_handle, timestamp, pid, version
@@ -209,7 +193,6 @@ class Ros2DataModel(DataModel):
         self.callback_symbols = pd.DataFrame.from_dict(self._callback_symbols)
         if self._callback_symbols:
             self.callback_symbols.set_index('callback_object', inplace=True, drop=True)
-        # Events (multiple instances, may not have a meaningful index)
         self.callback_instances = pd.DataFrame.from_dict(self._callback_instances)
 
     def print_data(self) -> None: