This discussion goes to the heart of the point of why you don't get your score right away even though you are taking a computerized test. Since different people take different tests, it would be unfair to just grade everyone based on a percentage of questions answered correctly. The USMLE therefore distributes scores along a curve, probably boiled down to each question individually, although I don't think anyone really knows for sure. This curve probably accounts for the phenomenon of many students leaving the test thinking they failed (and not just the paranoid and annoying students who always think that they failed after every exam), only to find out that they didn't do half bad. Just do your best and remember that if you have absolutely no freaking clue about a given question, the odds aren't bad that most students don't know the answer either (SDN gunners excluded).