docs: private double calculatePartitionQuality (ArrayList<Instance>[] docs,
  			HashSparseVector[] centroid) {
  		double quality = 0.0;
  
  		for (int i = 0; i < docs.length; ++i) {
  			quality += this.calculateClusterQuality(docs[i], centroid[i]);
  		}
  		return quality;
  	}