Success rates
Ah, back in my day zzzzzzzzzzz, out of 60 on my course NATS got 15 valid controllers, 25% success is fairly accurate but as many posts have said you can either do it or you can't. Every student that goes through any ATC training establishment has been identified as having the potential to become an ATCO but as any valid ATCO knows, that does not mean they will all make it in the end. Its a long road to validation but you will reap the rewards if successful. Good luck!