code: 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']