How do you measure output and what do you compare it to? Measuring output sounds like an ideal way to evaluate employees, but also sounds pretty challenging in

How do you measure output and what do you compare it to? Measuring output sounds like an ideal way to evaluate employees, but also sounds pretty challenging in and of itself. To be clear, identifying a poor engineer seems simple enough. But how do you tell the difference between a bright engineer working on a hard problem and a bright engineer being lazy? How long a task "should" take is often totally unknown. After all, if a programming task is so routine and well defined that it can be accurately estimated, that is probably good indication that this task can be automated, so we shouldn't be regularly doing such a task.