public function getCoefficientOfVariation($Serie)
    {
        if (isset($this->Data["Series"][$Serie])) {
            $Average = $this->getSerieAverage($Serie);
            $StandardDeviation = $this->getStandardDeviation($Serie);

            if ($StandardDeviation != 0) {
                return $StandardDeviation / $Average;
            }
        }
        return null;
    }