@Override
 public int compare(Passage o1, Passage o2) {
   String qid1 = o1.getQuestion().getId();
   String qid2 = o2.getQuestion().getId();
   int tmp = qid1.compareTo(qid2);
   if (tmp != 0) return tmp; // question id in ascending order
   Double score1 = o1.getScore();
   Double score2 = o2.getScore();
   return score2.compareTo(score1); // score in descending order
 }