public function getWeightVariance(): float
    {
        $mean = $this->getMeanWeight();

        $weightVariance = 0;
        /** @var PackedBox $box */
        foreach ($this->list as $box) {
            $weightVariance += ($box->getWeight() - $mean) ** 2;
        }

        return round($weightVariance / count($this->list), 1);
    }