def _get_kdjd(cls, df, n_days):
        """ Get the D of KDJ

        D = 2/3 × (prev. D) +1/3 × (curr. K)
        2/3 and 1/3 are the smooth parameters.
        :param df: data
        :param n_days: calculation range
        :return: None
        """
        k_column = 'kdjk_{}'.format(n_days)
        d_column = 'kdjd_{}'.format(n_days)
        df[d_column] = list(cls._calc_kd(df.get(k_column)))