src/com/chenlb/mmseg4j/Chunk.java
public double getVariance() {
		if(variance < 0) {
			double sum = 0;
			for(Word word : words) {
				if(word != null) {
					sum += Math.pow(word.getLength()-getAvgLen(), 2);
				}
			}
			variance = sum/getCount();
		}
		return variance;
	}
public double getVariance() {
		if(variance < 0) {
			double sum = 0;
			for(Word word : words) {
				if(word != null) {
					sum += Math.pow(word.getLength()-getAvgLen(), 2);
				}
			}
			variance = sum/getCount();
		}
		return variance;
	}
Variance of Word Lengths 就是 标准差的平方