public function stdDev(){
		
		Number::setScale(static::NUMERIC_FUNCTIONS_SCALE);
		
		$mean = $this->average();
		$squares = new static();
		
		foreach($this->_Arr as $num){
			$squares->add(Number::pow(Number::sub(\strval($num), $mean), '2'));
		}
		
		return Number::sqrt($squares->average());
		
	}