Over the last fifteen years a significant body of research has demonstrated that each of us is a disturbingly unreliable rater of other people’s performance. The effect that ruins our ability to rate others has a name: the Idiosyncratic Rater Effect, which tells us that my rating of you on a quality such as “potential” is driven not by who you are, but instead by my own idiosyncrasies—how I define “potential,” how much of it I think I have, how tough a rater I usually am. This effect is resilient — no amount of training seems able to lessen it. And it is large — on average, 61% of my rating of you is a reflection of me.
In other words, when I rate you, on anything, my rating reveals to the world far more about me than it does about you.
Marcus Buckingham writing in the Harvard Business Review