I have in the past worked in many civil service roles when performance appraisal has been brought in - every time it has caused anxiety, and for EVERY job EVERYONE has said- how can MY performance be measured, results depend on so many variables - not just me, "customers", colleagues, but department wide etc, etc, etc.....
ultimately performance related pay has been implemented, ultimately fears have been assuaged and people have just got on with it - WHY?
Because you will find that performance is actually MEASURED as an individual thing - Every year/term - whatever would be appropriate in the situation, an individual Performance appraisal is done with immediate management, a set of targets - SPECIFIC to the person concerned - are set for the next time defined period, the targets, means of attaining them, and the measurable criteria are set AND AGREED - so you have to perform well to your individual plan...
The system works... people start setting realistic, achievable, measurable targets and see where their strengths are and where their weaknesses can be targeted with training or additional input from management...
You should always have some targets which are mainstream and required by all, some which are to be striven for.... the way you complete your personal objectives is measurable and determines a "grade" - usually something like.... unsatisfactory, requires a little work, satisfactory, performing above expected, outstanding..
I can understand the anxiety about PRP but in every case (3 different departments) I have encountered it has made a difference in the workplace.