code: def compute_response_vowel_duration(self, prefix):
          
          durations = []
          for word in self.full_timed_response:
              if word.phones:
                  for phone in word.phones:
                      if phone.string in self.vowels:
                          durations.append(phone.end - phone.start)
  
          self.measures[prefix + 'response_vowel_duration_mean'] = get_mean(durations) \
              if len(durations) > 0 else 'NA'
  
          if not self.quiet:
              print "Mean response vowel duration:", self.measures[prefix + 'response_vowel_duration_mean']
docs: Computes mean vowel duration in entire response.
  
          :param str prefix: Prefix for the key entry in self.measures.
  
          Adds the following measures to the self.measures dictionary:
              - TIMING_(similarity_measure)_(collection_type)_response_vowel_duration_mean: average
                  vowel duration of all vowels in the response.