I'm a product of the 80's aircrew training system. At a guess, I would say about 35% were chopped between day one and combat ready status. Some of these were at IOT, then basics, then advanced, then OCU. Generally, there would be a remedial package of training followed by a recourse. Two strikes of failure put individuals on a very fine knife edge between further training or out.
My impression was that this process softened during the 90's. The OASC selection process was deemed to be faultless and therefore failures in training were not due to students not being capable, it was the fault of the training package. The result was to keep training people until they eventually passed.
As a former OCU instructor, I have to honestly say that some of the newly badged students coming through would not have made it in my day. The very real threat of being chopped ultimately led to survival of the fittest. Once the word 'chop' was removed from the training lexicon, students relaxed far too early. Of course, there were many quality students who would have always made the grade - but there were a few who ultimately were always going to struggle.