def METHOD_NAME(self):
    filter_age = None
    i = 0
    for file in self.files_input.values():
        df = pm.extract_data(
            file, region_spec='ID_County',
            column='Synthetic data',
            date=dt.date(2021, 11, 18),
            filters={'ID_State': None,
                     'Age_RKI': filter_age},
            file_format=self.file_format)
        if i == 0:
            dfs_all = pd.DataFrame(df.iloc[:, 0])
        dfs_all[df.columns[-1] + ' ' + str(i)] = df[df.columns[-1]]
        i += 1
    data_columns = dfs_all.columns[1:]
    assert len(data_columns) == 2