private double standardMoment(int n) {
		if (n < 0) throw new IllegalArgumentException();
		if (n == 0) return 1.0;
		if (n == 1) return 0.0;
		if (n == 2) return 1.0;
		makeValid();
		double sigma = stdDev();
		double sigmaN = sigma;
		for (int i = 1; i < n; i++) sigmaN *= sigma;
		return centralMoments[n] / sigmaN;
	}