How they're useful is debatable. But if you had a hundred people take an IQ test, and divided them into quartiles, you would notice some clear and strong diffe

How they're useful is debatable. But if you had a hundred people take an IQ test, and divided them into quartiles, you would notice some clear and strong differences between the groups, in a lot of different aspects. What to do about the differences is an exercise left for the reader.