The error of central tendency. Some raters do not like to rate employees as effective or ineffective, and so they distort the ratings to make to each employee appear average. On rating forms, this distortion causes evaluation to avoid checking extremes, such as very poor or excellent. Instead, they place their marks near the center of the rating sheet. Thus the term error of central tendency has been applied to this bias. HR departments sometimes unintentionally encourage this behavior by requiring raters to provide written justification of extremely high or low ratings.