code: def average(self):
          
          rslt = self._runQuery('SELECT', 'AVG(%s)' % (self._queryTarget,)) or [(0,)]
          assert len(rslt) == 1, 'more than one result: %r' % (rslt,)
          return rslt[0][0]
docs: Return the average value (as defined by the AVG implementation in the
          database) of the values specified by this query.
  
          Note: for non-numeric column types the result of this method will be
          nonsensical.
  
          @return: a L{float} representing the 'average' value of this column.