Be able to filter questions by difficulty

When it comes to studying, sometimes in your sets there are some 'plug and chug' type questions that are considered 'easy' and have a high percentage of students getting them right.  A basic appendicitis presentation, for example.  I think a cool feature would be to be able to sort questions by difficulty (based on percentage who answer right).  You can review your questions based on difficulty already (easy, medium, hard).  Perhaps you want to knock out all the 'easy' ones first as a warm up.  Or you only want to do 'hard' questions because you want to focus your studying on the perceived 'harder' topics/questions.  I think this would make the product more valuable to those who want to really narrow down what they're studying.