Thus it is important to distinguish between errors of central tendency and errors of range restriction (Saal , Downey ,& Lahey 1980). Central tendency errors involve a restriction of range,but only to the center or middle of the scale. Range restriction errors may involve such clustering around any point on a scale , often in combination with leniency errors at the very top. what is distinctive in both cases is a failure to distinguish real performance differences , either intentionally or because of insufficient attention