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.

def compute_response_vowel_duration(self, prefix):
        """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.
        """
        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']