You missed david's point. You're not supposed to criticize the person (ex. "You are sloppy!"). You're supposed to criticize the person's work (ex. "Your work is sloppy!").

In my experience, criticizing the person just drags down their morale and usually doesn't motivate them, whereas criticizing their work sometimes motivates them to improve their work (it can also sometimes drag down morale as well, depending on how you go about it).