public function quartile(int $index): Quartile
    {
        switch ($index) {
            case 0:
                return $this->min;
            case 1:
                return $this->firstQuartile;
            case 2:
                return $this->median;
            case 3:
                return $this->thirdQuartile;
            case 4:
                return $this->max;
        }

        throw new OutOfRangeException;
    }