diff --git a/openstack_qa_tools/collectors/_delta.py b/openstack_qa_tools/collectors/_delta.py
new file mode 100644
index 0000000..e824d65
--- /dev/null
+++ b/openstack_qa_tools/collectors/_delta.py
@@ -0,0 +1,46 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import json
+
+
+def delta(previous, current):
+    product = {}
+    seen = set()
+
+    # Old keys
+    for k, v in previous.items():
+        if k not in current:
+            continue
+        newv = current[k]
+        if type(v) is not type(newv):
+            raise ValueError(
+                'Type of key %s changed from %s to %s' % (k,
+                                                          type(v),
+                                                          type(newv)))
+        if isinstance(v, int) or isinstance(v, float):
+            product[k] = newv - v
+        elif isinstance(v, dict):
+            product[k] = delta(v, newv)
+        else:
+            raise ValueError('Only mappings of numbers are understood')
+        seen.add(k)
+    # New keys
+    for k in set(current.keys()) - seen:
+        product[k] = current[k]
+    return product
+
+
+def delta_with_file(previous_path, current_data):
+    with open(previous_path) as previous_file:
+        previous_data = json.loads(previous_file.read())
+    return delta(previous_data, current_data)
diff --git a/openstack_qa_tools/tests/test_collectors_delta.py b/openstack_qa_tools/tests/test_collectors_delta.py
new file mode 100644
index 0000000..fa4dc9e
--- /dev/null
+++ b/openstack_qa_tools/tests/test_collectors_delta.py
@@ -0,0 +1,71 @@
+# -*- coding: utf-8 -*-
+
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+"""
+test_delta
+----------------------------------
+
+Tests for `openstack_qa_tools.collectors._delta`
+"""
+
+import testscenarios
+
+from openstack_qa_tools.collectors import _delta
+from openstack_qa_tools.tests import base
+
+
+class TestOSQATDelta(testscenarios.WithScenarios, base.TestCase):
+
+    scenarios = [
+        ('add1', dict(
+            previous={'zoo': {'aardvark': 9, 'zebra': 0}},
+            current={'zoo': {'aardvark': 12, 'zebra': 0}},
+            expected={'zoo': {'aardvark': 3, 'zebra': 0}})),
+        ('newkey', dict(
+            previous={'zoo': {'bee': 0}},
+            current={'lake': {'trout': 1}, 'zoo': {'bee': 5}},
+            expected={'lake': {'trout': 1}, 'zoo': {'bee': 5}})),
+        ('delkey', dict(
+            previous={'zoo': {'cat': 99}},
+            current={},
+            expected={})),
+        ('newvar', dict(
+            previous={'zoo': {'dog': 9}},
+            current={'zoo': {'dog': 9, 'ocelot': 2}},
+            expected={'zoo': {'dog': 0, 'ocelot': 2}})),
+        ('delvar', dict(
+            previous={'zoo': {'elephant': 1000, 'bear': 1}},
+            current={'zoo': {'elephant': 1000}},
+            expected={'zoo': {'elephant': 0}})),
+        ('stringval', dict(
+            previous={'zoo': 'foo'},
+            current={'zoo': 0},
+            expected=ValueError)),
+        ('newstrval', dict(
+            previous={'zoo': {'giraffe': 100}},
+            current={'zoo': {'giraffe': 'tall'}},
+            expected=ValueError)),
+        ('changetype', dict(
+            previous={'zoo': {'horse': 7}},
+            current={'zoo': 15},
+            expected=ValueError)),
+    ]
+
+    def test_delta(self):
+        if self.expected is ValueError:
+            self.assertRaises(ValueError, _delta.delta, self.previous,
+                              self.current)
+        else:
+            self.assertEqual(self.expected, _delta.delta(self.previous,
+                                                         self.current))