For decades, teachers, managers and parents have assumed that the performance of students and employees fits what’s known as the bell curve — in most activities, we expect a few people to be very good, a few people to be very bad and most people to be average.
This isn’t the first time I’ve found that someone is shocked, shocked to find that you have non-normal distributions after you’ve run your sample through some kind of discriminator. Managers don’t tend to hire the unqualified. Students that aren’t college material tend not to go to college, or drop out. If you aren’t good enough to be competitive at a sport you won’t be on the team. Once you have limited your sample in a way that introduces a bias, don’t automatically expect the distribution to be normal.