There are a lot of good reasons why people can suck at their jobs. One of the biggest may be you, their manager. Yet it is very possible that the entire rest of your team may have had a fantastic year for the very same reason – you, their manager.
So is the problem you or your poor performing employee? Yes and Yes or possibly No and No or maybe Neither. Some situations may require you to back away from assigning blame and instead look at the environment.
