Computes the average pairwise similarity score between all pairs of Units.

        The pairwise similarity is calculated as the sum of similarity scores for all pairwise
        word pairs in a response -- except any pair composed of a word and
        itself -- divided by the total number of words in an attempt. I.e.,
        the mean similarity for all pairwise word pairs.

        Adds the following measures to the self.measures dictionary:
            - COLLECTION_collection_pairwise_similarity_score_mean: mean of pairwise similarity scores

        .. todo: divide by (count-1)?

def compute_pairwise_similarity_score(self):
        """Computes the average pairwise similarity score between all pairs of Units.

        The pairwise similarity is calculated as the sum of similarity scores for all pairwise
        word pairs in a response -- except any pair composed of a word and
        itself -- divided by the total number of words in an attempt. I.e.,
        the mean similarity for all pairwise word pairs.

        Adds the following measures to the self.measures dictionary:
            - COLLECTION_collection_pairwise_similarity_score_mean: mean of pairwise similarity scores

        .. todo: divide by (count-1)?
        """
        pairs = []
        all_scores = []
        for i, unit in enumerate(self.parsed_response):
            for j, other_unit in enumerate(self.parsed_response):
                if i != j:
                    pair = (i, j)
                    rev_pair = (j, i)
                    if pair not in pairs and rev_pair not in pairs:
                        score = self.compute_similarity_score(unit, other_unit)
                        pairs.append(pair)
                        pairs.append(rev_pair)
                        all_scores.append(score)

        #remove any "same word" from the mean
        all_scores = [i for i in all_scores if i != self.same_word_similarity]
        self.measures["COLLECTION_" + self.current_similarity_measure + "_pairwise_similarity_score_mean"] = get_mean(
            all_scores) \
            if len(pairs) > 0 else 'NA'