def _normalize(df):
    """
    Normalise the county data i.e. strip spaces and convert to upper case, and finally sort alphabetically.

    :param df: input data frame with county column
    :type df: pandas.DataFrame

    :return: normalised data
    :rtype: pandas.DataFrame
    """
    df.sort_values(by='county', inplace=True)
    df['county'] = df['county'].str.upper()
    df['county'] = df['county'].str.strip()

    return df