teachers. The issue comes from the fact that teachers deal with complicated, moody, immature human beings. This means that it's hard to tie pay to student performance (i.e. evidence of learning in a classroom or how effective a teacher is) because of how multi-faceted humans are. I've had smart students who are going through tough times (i.e. parent divorce, breaking up with boyfriend/girlfriend, car problems) that affect their performance.

Bottom line, it's really hard to evaluate true teacher effectiveness without having almost daily observations.