code: public Double getVariance() {
  		if (counter == 0) {
  			return Double.NaN;
  		} else {
  			final double mean = computeMean();
  			final double meanSquare = mean * mean;
  			final double squareMean = ((double) squareTotal) / counter;
  			return squareMean - meanSquare;
  		}
  	}
docs: Compute variance.
  
  @return variance