Constructive criticism from your boss can be a gift in the way that a free gym membership can be a gift: Even if you didn't want it, it can benefit you greatly.
Count yourself fortunate if you have a boss who is thoughtful enough to let you know early on that your performance isn't where it should be. Many bosses simply let a poor performer slide until it's too late and their job is at risk. If your boss pulls you aside to let you know that you're just not cutting it, consider the following:
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Don't get defensive. Being criticized can bring out the feral cat in anyone—the back arches, the fangs come out, the hair bristles (all figuratively, of course). While it's natural to become defensive when being called out for poor performance at work, it's an impulse to fight. Defensiveness closes your mind to legitimate criticism. And it will hurt you in the long run—when you get fired for not improving your performance despite having been warned.
Understand the problem. You choked down your initial defensiveness. You opened your mind to the criticism. Now the most important things you need to know are how exactly you are falling short and what you need to do to get your performance up to par. Simply knowing that your boss is unsatisfied won't do. For criticism to be constructive, you need specifics.
Address the problem. If you've adhered to the first two suggestions, this should be the easy part (assuming that the expectations your boss has for you are reasonable). Come up with a concrete plan to address the issues your boss raised. This can be as simple as dedicating more time to a particular aspect of your job, learning to become more organized, or working to develop some job-related skill.
Seek more feedback. If your boss doesn't deliver subsequent criticism, don't assume everything is OK. Make sure you're going in the right direction by seeking additional feedback. But don't be a pest. Figure out the best way to communicate: Schedule periodic meetings, or communicate via phone or email. Let your boss know what you're doing and ask whether it is in line with his or her expectations.
Luke Roney is content manager for CareerBliss, an online career community dedicated to helping people find happiness in the workplace. Check out CareerBliss for millions of job listings, company reviews, salary information and a free career happiness assessment.