public function getStandardDeviation(array $data = []): ?float 	{ 		$n = count($data);  		if ($n < 1) { 			return null; 		}  		$mean = array_sum($data) / $n;  		$distance_sum = 0;  		foreach ($data as $i) { 			$distance_sum += ($i - $mean) ** 2; 		}  		$variance = $distance_sum / $n;  		return sqrt($variance); 	}