def get_most_and_least_freq_words(frequency_dict_):
    """
    This function returns the most and least frequent words that can be considered noise.

    :param frequency_dict_: Dictionary that holds the frequency information of words.
    :type frequency_dict_: dict
    :return: Words that are frequent in these lexicons.
    :rtype: set
    """
    size = len(frequency_dict_)
    frequency_el = int(size / 100)

    most_freq = frequency_dict_.most_common(frequency_el)
    least_freq = frequency_dict_.most_common()[:-frequency_el - 1:-1]

    union_set = set([k[0] for k in most_freq]).union(set(k[0] for k in least_freq))

    return union_set