I'll also add that it's wrong to assume that HR get it 50% wrong in selection based on the assumption that 50% of trainees fail. Trainees who are selected are simply 'suitable for training', not guaranteed potential rated controllers. Given the very small percentage of the general population that have the spatial reasoning, problem solving and processing speed skills required to make a successful ATC, the fact that HR can give us 5 ATC's for every 10 trainees that start (on the 50% assumption) is a pretty good strike rate tbh.