Megan McArdle argues that teacher unions kill initiatives to tie compensation to performance because

unions are always going to be looking for the simplest, least subjective metrics by which to measure their members. Furthermore, they will be looking for metrics which are not under the control of the other side. The school board cannot change how many years you have in service, or whether or not you have a degree. But it can change the curriculum, or the tests… Unions are set up to minimize frictions and maximize benefits for the bottom 55%. That’s how they work everywhere–in schools, and out. That’s how they have to work. No amount of cajoling, no number of white papers, is going to change that.