Interesting - so if 80-90% of variance is due to home environment, it seems like schools are just a minor concern (assuming the remaining 10-20% is all due to schools). Improving schools by 10% only improves students by 1-2%! Unless such improvements are cheap, they are almost certainly not cost effective.
It also means that international comparisons of student performance are almost certainly worthless - we aren't comparing school performance, we are simply comparing student quality.
Incidentally, you are wrong on your changes since 1969. The poor have gotten vastly richer since then. In 1969, the bottom 10% of the country didn't even have flush toilets.
"Improving schools by 10% only improves students by 1-2%"
The fact that 90% of the gap between high-SES students and low-SES students is attributable to home factors doesn't tell us what percent of their total knowledge students gain at school verse at home.
And remember that percent is very different for high-SES and low-SES students-- the study says that high-SES students learn 3.5-4x times faster in school than at home. Whereas low-SES students learn 16x times faster in school than at home. And the vast majority of students are low- or middle-SES.
So while improving schools by 10% probably wouldn't be a good way to close the gap between high-SES and low-SES students, it might still be cost effective at increasing their total knowledge and ability.
Also, when I said the poor have gotten poorer, what I meant was that the gap between the rich and poor has gotten larger, because again that's what this study is about.
The fact that 90% of the gap between high-SES students and low-SES students is attributable to home factors doesn't tell us what percent of their total knowledge students gain at school verse at home.
Sorry, I guess I misunderstood you then. I asked "what % of test score variance is caused by the home environment", and you said "80-90%". So is it fair to say that we really don't know what factors predict student achievement, all we really have is a possible explanation of one particular delta?
In that case, the story we are discussing is perhaps more useful than you initially thought.
Also, when I said the poor have gotten poorer, what I meant was that the gap between the rich and poor has gotten larger, because again that's what this study is about.
This is also unclear. In terms of dollars, sure, but not necessarily in terms of living conditions. In 1970, the rich had servants while the poor had outhouses. In 2010, the rich have iPhones while the poor have Droids (sometimes even a dumb phone without data).
It also means that international comparisons of student performance are almost certainly worthless - we aren't comparing school performance, we are simply comparing student quality.
Incidentally, you are wrong on your changes since 1969. The poor have gotten vastly richer since then. In 1969, the bottom 10% of the country didn't even have flush toilets.