Based on watching a lot of people come and go in our shop and our engineering dept, I would say that about half of the people hired on average just did not work out.
Mostly the shop people that did not work out were as much lack of internal motivation as anything else. Most of these guys did not last long as we usually hire the ones no one knows or has any confidence in thru temp agencies. I have seen guys that spent most of a day chatting or texting and then wonder why a few days later they were told their services were no longer needed.
Most of the engineers that did not work out were due to lack of skills they claimed to have. It usually took a while for the bosses to figure them out, although most of the guys that had to work with them figured them out pretty quick.
One of the problems we have run into is that things have been so bad for so long in our area that basically any desirable employees have left town. It is very hard to find people locally, and even harder to attract qualified people to move here, since it is tough for our company to offer moving packages, especially for shop people.