public double variance() {
		double var = NaN;
		if (_n == 1L) {
			var = _m2.value();
		} else if (_n > 1L) {
			var = _m2.value()/(_n - 1.0);
		}

		return var;
	}
Return the variance of values recorded, or {@code Double.NaN} if no
values have been recorded.

@return the variance of values, or {@code NaN} if none
jenetics/src/main/java/io/jenetics/stat/MomentStatistics.java