def get_lowest_numeric_score(self):
        """Gets the lowest number in a numeric grading system.

        return: (decimal) - the lowest number
        raise:  IllegalState - ``is_based_on_grades()`` is ``true``
        *compliance: mandatory -- This method must be implemented.*

        """
        if self.is_based_on_grades():
            raise errors.IllegalState('This GradeSystem is based on grades')
        if self._my_map['lowestNumericScore'] is None:
            return None
        else:
            return Decimal(str(self._my_map['lowestNumericScore']))