I joined no.54 course in March 1983. All 16 made it through the ratings courses. The 7 who went to Scottish validated. I think perhaps three who went to London failed to make it.
The recruitment criteria is critical. I think the basis of ours was how keen you were on aviation, and not how well you had done at school. Only one had a degree. A third in chemistry.
We all wanted to do it, and half were assistants (class to class promotion)
A few years later the recruitment was based on other criteria, probably more acedemic success. Guess what, significant drop in validation rate.
So to my simple mind, you select those who have a passion for aviation.