I'm afraid some here are missing the point.
The stage 1 selection tests do not and cannot select those who will make good ATCOs and discard those who will not.
What they do accomplish, however, is test according to a set standard in areas of mental calculation and agility in which it has been shown that successful ATCOs perform highly. Therefore statistically, those who attain a good mark in all the disciplines have a higher chance of successfully completing the training and making a good ATCO.
There is continual feedback into the system as results are analysed and those selected by the system are tracked through to validation. Changes to the selection system are experimented upon. More tests might be added to stage one next year.
I do not believe there is a selection process extant that has a 100% success rate, be it ATC or anything else.