public function addObservation( $x ) {
		$x = (float)$x;

		$this->min = min( $this->min, $x );
		$this->max = max( $this->max, $x );

		$n1 = $this->n;
		$this->n += 1;
		$delta = $x - $this->m1;
		$delta_n = $delta / $this->n;
		$this->m1 += $delta_n;
		$this->m2 += $delta * $delta_n * $n1;
	}