How often do professors regret accepting particular graduate students (i.e., "bad hires")? Completely out of curiosity, I was wondering how often PIs have to deal with "bad hires" in their lab/group? I sometimes hear the PI/trainee relationship be compared to marriages, where half ends in a divorce. Is there actual truth to this? There are also posts like these that reinforce this:

Tales of postdocs past: what did I learn?

As profs, have you ever accepted grad students only to regret it later? Does this happen often and how do you deal with it? Are problems typically that they cannot do the work, or that their personality is just not a good fit? I think for the most part PIs I've met handle these professionally, but I do wonder how they actually feel about the situation.
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)