Key discussion here about value-added assessment of INDIVIDUAL teachers. Note these comments specifically:
"And, unfortunately, they are not alone. I hear people – including policymakers – advocate constantly for the use of value-added in teacher evaluations or other high-stakes decisions by saying that 'research shows' that there are huge differences between 'good' and 'bad' teachers.
"This overall variation is a very important finding, but for policy purposes, it doesn’t necessarily mean that we can differentiate between the good, the bad, and the average at the level of individual teachers. How we should do so is an open question.
"Conflating the importance of teacher quality with the ability to measure it carries the risk of underemphasizing all the methodological and implementation details – such as random error, model selection, and data verification – that will determine whether value-added plays a productive role in education policy."