@staticmethod
def _zscore(y, x, window):
    """
        Ratio Moving Average Differential Method
        """
    ratio = y / x
    rolling_ratio = ratio.rolling(window='{}D'.format(window), min_periods=
        int(window / 2))
    ratio_ma = rolling_ratio.mean()
    ratio_std = rolling_ratio.std()
    spread = (ratio - ratio_ma) / ratio_std
    return spread.dropna()
