Bias is the tendency to allow individual differences such as age, race, and sex to affect the appraisal ratings employees receive.
The number of things that can lead to bias during appraisals is limitless. One study focused on the rater’s personality. Raters who scored higher on “conscientiousness” tended to give their peers lower ratings they were stricter, in other words; those scoring higher on “agreeableness” gave higher ratings- they were more lenient.