@ddljohn-- If the employee aims to be factual, biases can be prevented.
I just filled out my first personal performance evaluation at work this week. It wasn't as difficult as I thought it would be. I just stuck to the facts and gave many examples of my work and efforts. I also mentioned a mistake I made earlier in the year which my manager already knows about. I elaborated and wrote about how I will avoid such mistakes in the future. We had a brief discussion about my evaluation later with my manager. It was a pleasant and informative discussion. It turns out my manager rated me as well and his rating was higher than my rating for myself! Next time, I'm going to be less modest.