@staticmethod
def filter_by_rat(df, threshold=3):
    """
        Filter the DataFrame keeping in only ratings greater equal than threshold
        ------------
        :param df: pandas DataFrame
        :param threshold: minimum considered value in df
        :return: the pandas DataFrame with filtered ratings
        """
    rat_col = list(df)[2]
    df = df[df[rat_col] >= threshold]
    return df
