Judging what works in education

Another NPR story, this time about a researcher in education from Melbourne, yet I am not familiar with him.   As with all of education research, it may be that some of his claims are debatable, but I strongly suspect this one is right:

Many education reformers tout school choice as a tool for parent empowerment and school improvement through competitive pressure. But Hattie says his research shows that once you account for the economic background of students, private schools offer no significant advantages on average. As for charter schools? "The effect of charter schools, for example, across three meta-analyses based on 246 studies is a minuscule .07," he writes.

On the other hand, I don't quite understand how you study this at all:

Putting televisions in the classroom, on the other hand, has an average negative impact of -0.18. Holding students back a grade really does hold students back, with an effect of -0.16.

How do you judge how the child would have done if they had not been "held back"?  

Update:  I also note that Naplan results in Australia indicate that having a mother born overseas is a good way to stay above the average. Bit hard to address that in your education system, though....