public function get_last_analysed_time($qubaids, $whichtries) {
        global $DB;

        $timemodified = time() - self::TIME_TO_CACHE;
        return $DB->get_field_select('question_response_analysis', 'timemodified',
                                     'hashcode = ? AND whichtries = ? AND questionid = ? AND timemodified > ?',
                                     array($qubaids->get_hash_code(), $whichtries, $this->questiondata->id, $timemodified),
                                     IGNORE_MULTIPLE);
    }