Measuring "outcome" in education is very hard, specially when deciding pay, as it creates perverse incentives. For example, teachers optimizing for test results

Measuring "outcome" in education is very hard, specially when deciding pay, as it creates perverse incentives. For example, teachers optimizing for test results instead of actual education, disadvantaged classes getting less attention (because results will be worse so it will affect teacher's evaluation)... Evaluating teachers based on outcome is something that sounds good at first but it's plagued with problems no matter how you try to do it.