public int compare(Instance s1, Instance s2) 
	{
		if(!s1.containsFeature(fid)) return 1;
		if(!s2.containsFeature(fid)) return -1;
		
		if(s1.getFeature(fid) - s2.getFeature(fid) > 0)
			return 1;
		if(s1.getFeature(fid) - s2.getFeature(fid) < 0)
			return -1;
		return 0;
	}