diff --git a/mis_builder/models/aggregate.py b/mis_builder/models/aggregate.py
new file mode 100644
index 00000000..4788c4c2
--- /dev/null
+++ b/mis_builder/models/aggregate.py
@@ -0,0 +1,149 @@
+# -*- encoding: utf-8 -*-
+##############################################################################
+#
+#    mis_builder module for Odoo, Management Information System Builder
+#    Copyright (C) 2014-2015 ACSONE SA/NV (<http://acsone.eu>)
+#
+#    This file is a part of mis_builder
+#
+#    mis_builder is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License v3 or later
+#    as published by the Free Software Foundation, either version 3 of the
+#    License, or (at your option) any later version.
+#
+#    mis_builder is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License v3 or later for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    v3 or later along with this program.
+#    If not, see <http://www.gnu.org/licenses/>.
+#
+##############################################################################
+
+
+def _sum(l):
+    """ Same as stdlib sum but returns None instead of 0
+    in case of empty sequence.
+
+    >>> sum([1])
+    1
+    >>> _sum([1])
+    1
+    >>> sum([1, 2])
+    3
+    >>> _sum([1, 2])
+    3
+    >>> sum([])
+    0
+    >>> _sum([])
+    """
+    if not l:
+        return None
+    return sum(l)
+
+
+def _avg(l):
+    """ Arithmetic mean of a sequence. Returns None in case of empty sequence.
+
+    >>> _avg([1])
+    1.0
+    >>> _avg([1, 2])
+    1.5
+    >>> _avg([])
+    """
+    if not l:
+        return None
+    return sum(l) / float(len(l))
+
+
+def _min(*args):
+    """ Same as stdlib min but returns None instead of exception
+    in case of empty sequence.
+
+    >>> min(1, 2)
+    1
+    >>> _min(1, 2)
+    1
+    >>> min([1, 2])
+    1
+    >>> _min([1, 2])
+    1
+    >>> min(1)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: 'int' object is not iterable
+    >>> _min(1)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: 'int' object is not iterable
+    >>> min([1])
+    1
+    >>> _min([1])
+    1
+    >>> min()
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: min expected 1 arguments, got 0
+    >>> _min()
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: min expected 1 arguments, got 0
+    >>> min([])
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    ValueError: min() arg is an empty sequence
+    >>> _min([])
+    """
+    if len(args) == 1 and not args[0]:
+        return None
+    return min(*args)
+
+
+def _max(*args):
+    """ Same as stdlib max but returns None instead of exception
+    in case of empty sequence.
+
+    >>> max(1, 2)
+    2
+    >>> _max(1, 2)
+    2
+    >>> max([1, 2])
+    2
+    >>> _max([1, 2])
+    2
+    >>> max(1)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: 'int' object is not iterable
+    >>> _max(1)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: 'int' object is not iterable
+    >>> max([1])
+    1
+    >>> _max([1])
+    1
+    >>> max()
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: max expected 1 arguments, got 0
+    >>> _max()
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    TypeError: max expected 1 arguments, got 0
+    >>> max([])
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    ValueError: max() arg is an empty sequence
+    >>> _max([])
+    """
+    if len(args) == 1 and not args[0]:
+        return None
+    return max(*args)
+
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()
diff --git a/mis_builder/models/mis_builder.py b/mis_builder/models/mis_builder.py
index 109b9fdc..bb3f676d 100644
--- a/mis_builder/models/mis_builder.py
+++ b/mis_builder/models/mis_builder.py
@@ -35,6 +35,7 @@ from openerp import api, fields, models, _
 from openerp.tools.safe_eval import safe_eval
 
 from .aep import AccountingExpressionProcessor as AEP
+from .aggregate import _sum, _avg, _min, _max
 
 _logger = logging.getLogger(__name__)
 
@@ -69,30 +70,6 @@ def _is_valid_python_var(name):
     return re.match("[_A-Za-z][_a-zA-Z0-9]*$", name)
 
 
-def _sum(l):
-    if not l:
-        return None
-    return sum(l)
-
-
-def _avg(l):
-    if not l:
-        return None
-    return sum(l) / float(len(l))
-
-
-def _min(l):
-    if not l:
-        return None
-    return min(l)
-
-
-def _max(l):
-    if not l:
-        return None
-    return max(l)
-
-
 class MisReportKpi(models.Model):
     """ A KPI is an element (ie a line) of a MIS report.