How often do professors regret accepting particular graduate students (i.e., "bad hires")?
I've discussed this with a few line managers, I've hired a few people, and I've checked with headhunters. We all come to very similar numbers**.

Once you're doing the recruiting well, you can expect that for every five people you hire, on average one will be an extremely good hire, and three will do the job well enough. And the fifth will be regrettable, through some combination of lack of will, lack of ability, an absence of a shared understanding of what the role requires, &amp; personality clashes. You can try moving them to different roles, get them more training, work with them to find out what they need. Sometimes one of those things will work. Sometimes, none will.



** My primary dataset is academia. I got similar answers from the wider market for professionals (not manual workers)