It's not good enough to test only the people who have already passed a similar test. It's easy to get a nonsense negative correlation, like more experienced peo

It's not good enough to test only the people who have already passed a similar test. It's easy to get a nonsense negative correlation, like more experienced people performing worse in their job, because the people with less experience that nonetheless got hired must have had some other factor that made up for it. That can result in a negative correlation in the already-hired pool even if there's a positive correlation in the potential-hire pool. This is the same problem you see with people claiming that GPA or GRE scores have no or negative correlation with academic success.