Joel talks about software development, where the real impact of individual contributions is hard to measure. Any metric you set will be inaccurate, and thus harmful, because people will be motivated to maximize the metric, rather than real value. You just have to find people you can trust and empower, then trust them and empower them.

Now, healthy days is quite a good heuristic to evaluate a doctor's performance, don't you think? How do you think it can be gamed; i.e., what exploits can you think of?