def _get_atr(cls, df, window=None):
        """ Average True Range

        The average true range is an N-day smoothed moving average (SMMA) of
        the true range values.  Default to 14 days.
        https://en.wikipedia.org/wiki/Average_true_range

        :param df: data
        :return: None
        """
        if window is None:
            window = 14
            column_name = 'atr'
        else:
            window = int(window)
            column_name = 'atr_{}'.format(window)
        tr_smma_column = 'tr_{}_smma'.format(window)

        df[column_name] = df[tr_smma_column]
        del df[tr_smma_column]