There has long been a mystery in why the rapid growth of education in poor countries did not pay off in growth of production per worker, above all in Africa (best captured by a classic paper by Lant Pritchett, Where has all the education gone?, ungated here) Eric Hanushek at Stanford has been working for the past several years on test scores as a possible resolution of the puzzle. If education doesn't translate into higher test scores, then there is something else wrong along the way, which likely includes well-known problems like absent teachers and missing textbooks. He showed this picture in a 2008 paper, and he has a stream of papers since, all with coauthor Ludger Woessman.
Growth is growth of income per person 1960-2000. Both growth and test scores are measured "conditionally," that is how well they do relative to a country's initial educational enrollment and income in 1960.
Of course, test scores are a potentially sensitive subject, as some will think they are tests of intrinsic intelligence. Is this whole area of research racist?
Not necessarily, of course. Let's take racist stories of differing intelligence between nations off the table, and consider all the other factors that could be reflected in such widely varying test scores relative to educational enrollment and income.