central tendency bias
Can occur when managers are evaluating the performance of their staff (typically by using a 5-point scale) but fail to differentiate sufficiently between individuals. The tendency is to score everyone around the midpoint of the scale, and not use the extremes. The result of this is sometimes called the central tendency error. In order to counteract the central tendency bias, some organizations use a forced distribution, which obliges managers to place a certain percentage of employees at the top and bottom of the scale.