'Get the n most common words in questions
n=4600 ~= thresh 6'

def get_common_question_words(self, n):

    from nltk.tokenize import word_tokenize
    cnt = Counter()
    for q in self.questions:
        cnt.update(word_tokenize(q['question'].lower()))
    del cnt['?']
    ret = cnt.most_common(n)
    return [k[0] for k in ret]
