A picture is worth 10K words - but only those to describe the picture. Hardly any sets of 10K words can be adequately described with pictures. --Alan Perlis
Y = the contribution of the system they work within
[XY] = the interaction of the individual with the system
8 represents some measure of productivity, e.g., rate of errors, millions of dollars in profit, whatever you're measuring
The person who can solve for X is competent to rate people on their performance.
What to do instead of (destructively) rating people?
Build better systems for doing the work, make their work easier, give them psychological safety and job security so they can relax and enjoy their work and share better methods with each other.