public function feedbacks($level = null)
    {
        if ($level !== null) {
            if (!is_string($level)) {
                throw new InvalidArgumentException('The feedback level must be a string');
            }

            $level = $this->resolveFeedbackLevel($level);

            $subset = [];
            foreach ($this->feedbacks as $item) {
                if ($item['level'] === $level) {
                    $subset[] = $item;
                }
            }

            return $subset;
        }

        return $this->feedbacks;
    }