protected function clear_cached_data($qubaids) {
        global $DB;
        $DB->delete_records('quiz_statistics', array('hashcode' => $qubaids->get_hash_code()));
        $DB->delete_records('question_statistics', array('hashcode' => $qubaids->get_hash_code()));
        $DB->delete_records('question_response_analysis', array('hashcode' => $qubaids->get_hash_code()));
    }