The problem lies in the fact that all teachers are not teaching the same students and so it is hard to make fair comparisons. (capoliticalreview.com)
For fair comparison also, you'd want to compare the returns of an index fund or ETF, not just the index. (canadiancapitalist.com)
As repeatedly confirmed by empirical studies, it permits fair comparisons among teachers, schools, and school systems that serve differing student populations. (education-consumers.org)