A process for objective review of physician performance

How do you objectively evaluate physicians at reappointment. How do you establish a common ground for the evaluation process that still acknowledges acceptable differences in performance? Perhaps one physician has some difficulty with documentation and attendance at meetings, but has no quality problems clinically. Another physician may have good documentation and meeting attendance, but has some quality problems. Another physician has a behavior pattern that is disruptive, a few documentation problems, but excellent quality. Yet another physician is a marginal practitioner with major problems in several areas, including quality. Reappointment of these physicians might be extremely difficult, especially if the credentials committee is recently appointed and not familiar with the details of the performance data.