docs: 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;
  	}