$this->addFunction('stats_standard_deviation',
	function(array $a, $sample = false) {
		$n = count($a);
		if ($n === 0) {
			return 0;
		}
		if ($sample && $n === 1) {
			return 0;
		}
		$mean = array_sum($a) / $n;
		$carry = 0.0;
		foreach ($a as $val) {
			$d = ((double) $val) - $mean;
			$carry += $d * $d;
		};
		if ($sample) {
			--$n;
		}
		return sqrt($carry / $n);
	},
	['desc' => 'Среднеквадратичное отклонение.']
);