We have a discussion with every employee each 6 months. This discussion is open and we chat about topics that were learned, knowledge that was improved, etc. This affects salary rise (as well as good performance overall).
How are you estimate how much you would rise depending on employee performance? Do you have metrics or just by intuition?
Also, it's interesting what questions do you discuss in order to understand that somebody was improved since previous review. How many books have been read or something else?