That last statement is almost more interesting than the autopilot stuff. How do they determine who's "really" a good driver in order to correlate it with whatev

That last statement is almost more interesting than the autopilot stuff. How do they determine who's "really" a good driver in order to correlate it with whatever they're measuring that has a bimodal distribution? Is it possible to teach someone to change their driving style so that they're in the other lobe? If so do they become an actual better/worse driver?