In my own experience, and from what I have gathered from clients, colleagues, students, and friends, dealing successfully with those who don’t quite measure up to the standards you expect is one of the greatest challenges a manager can face.
One of the reasons for this is that, in a way, a poor performance shows that you have personally failed in some sense. Perhaps you hired or promoted this failing person, and you expected more; or your efforts to bring about change in the errant p