If employers do not require a(n) , performance appraisal ratings often do not match the normal distribution of a bell-shaped curve.

forced distribution

Explanation:

Based on the rest of the sentence it can be said that the missing term is forced distribution. This is a system that requires managers to evaluate each individual and rank them typically into one of three categories. These categories are excellent, good, and poor and allow managers to indicate if the employee should be terminated, is doing good, or is in-line for promotion as indicated in the graph below. This term is also known as the vitality curve or bell curve.