public static function create_new($definitionid, $raterid, $itemid) {
        global $DB;
        $instance = new stdClass();
        $instance->definitionid = $definitionid;
        $instance->raterid = $raterid;
        $instance->itemid = $itemid;
        $instance->status = self::INSTANCE_STATUS_INCOMPLETE;
        $instance->timemodified = time();
        $instance->feedbackformat = FORMAT_MOODLE;
        $instanceid = $DB->insert_record('grading_instances', $instance);
        return $instanceid;
    }