code: public double getKurtosis() {
  		double kurtosis = NaN;
  		if (_n > 3L) {
  			final double var = _m2.value()/(_n - 1);
  			if (_n <= 3L || var < 10E-20) {
  				kurtosis = 0.0;
  			} else {
  				kurtosis = (_n*(_n + 1.0)*_m4.value() -
  					3.0*_m2.value()*_m2.value()*(_n - 1.0))/
  					((_n - 1.0)*(_n - 2.0)*(_n - 3.0)*var*var);
  			}
  		}
  		return kurtosis;
  	}
docs: Return the kurtosis of values recorded, or {@code Double.NaN} if less
  than four values have been recorded.
  
  @see <a href="https://en.wikipedia.org/wiki/Kurtosis">Kurtosis</a>
  
  @return the kurtosis of values, or {@code NaN} if less than four values
  have been recorded