code: public double getSkewness() {
  		double skewness = NaN;
  		if (_n >= 3L) {
  			final double var = _m2.value()/(_n - 1.0);
  			if (var < 10E-20) {
  				skewness = 0.0d;
  			} else {
  				skewness = (_n*_m3.value())/
  						((_n - 1.0)*(_n - 2.0)*sqrt(var)*var);
  			}
  		}
  
  		return skewness;
  	}
docs: Return the skewness of values recorded, or {@code Double.NaN} if less
  than two values have been recorded.
  
  @see <a href="https://en.wikipedia.org/wiki/Skewness">Skewness</a>
  
  @return the skewness of values, or {@code NaN} if less than two values
  have been recorded