protected function _filterAnswersOnly($sourceSurveyId, array $answers)
    {
        $s = $sourceSurveyId . 'X';
        $l = strlen($s);

        $results = array();
        foreach ($answers as $key => $value) {
            if (substr($key, 0, $l) == $s) {
                $results[$key] = $value;
            }
        }

        return $results;
    }