To provide useful information for the decision maker, performance appraisals must be three things. Valid and reliable are two of the three things. What is the third?

Free from bias

Explanation:

In order to legitimately review and evaluate somebody's performance, a person should also be free from bias towards any of the employees.

When making a decision in the company, the prejudices should be put aside. The person that makes the evaluation should be extra cautious about psychological mechanisms that affect prejudicial, biased thinking - halo effect (making the decision on the first look), contrast (comparing people against each other, and not according to company standards), recency (only evaluating most recent behavior), etc.

To make the best possible decision, performance appraisals must be done as objectively as possible, taking into account all the possible biases that can make the difference.