def compute_within_collection_vowel_duration(self, prefix, no_singletons=False):
        if no_singletons:
            min_size = 2
        else:
            prefix += "no_singletons_"
            min_size = 1
        durations = []
        for cluster in self.collection_list:
            if len(cluster) >= min_size:
                for word in cluster:
                    word = self.full_timed_response[word.index_in_timed_response]
                    for phone in word.phones:
                        if phone.string in self.vowels:
                            durations.append(phone.end - phone.start)
        self.measures[prefix + 'within_collection_vowel_duration_mean'] = get_mean(durations) \
            if len(durations) > 0 else 'NA'
        if not self.quiet:
            if no_singletons:
                print "Mean within-" + self.current_similarity_measure + "-" + self.current_collection_type + \
                      " vowel duration, excluding singletons:", \
                    self.measures[prefix + 'within_collection_vowel_duration_mean']
            else:
                print "Mean within-" + self.current_similarity_measure + "-" + self.current_collection_type + \
                      " vowel duration, including singletons:", \
                    self.measures[prefix + 'within_collection_vowel_duration_mean']