def slope_percentile(self, date, mag):
        date_diff = date[1:] - date[:len(date) - 1]
        mag_diff = mag[1:] - mag[:len(mag) - 1]
        index = np.where(mag_diff != 0.)
        date_diff = date_diff[index]
        mag_diff = mag_diff[index]
        slope = date_diff / mag_diff
        percentile_10 = np.percentile(slope, 10.)
        percentile_90 = np.percentile(slope, 90.)
        return percentile_10, percentile_90