Merit Pay can't work

Dan Willingham does it again. This time he discusses why Merit Pay for Teachers won't work.

He ends with the following: "Merit pay can't work until there's a way to measure teacher performance that's fair."

I feel that the problem is not about fairness. Merit pay can't work until we have a way to measure the performance of the teachers directly instead of using indirect measures, like children's test scores or community income levels.