I work in healthcare and the model often used is Brenners Novice to Expert. This looks at the development of an individual in their practice. While a great model since it allows one to compare themselves to themselves and looking for improvement, it also promotes team work. Of course this is a little difficult to apply many software firms. Another model is using a 1-5 scale, where 5 is exceptional, 1 is unsatistifactory, 3 meets criteria, 4 is exceeds criteria, and then they tally these for whatever metrics used and divide to get an average.
Comparing staff to each other does not develop team work and only works in competitive environments like sales where you want people to outdo each other.