Shown on the x-axis are Hanushek and Woessmann's combination of scores from international student achievement tests.
The scores are standardized to the PISA test scale, so that the OECD countries have a mean of 500 and a standard deviation of 100.
The test scores are here plotted agains GDP per capita in 2016, but the test scores are not from a particular year, but instead are the average of all standardized math and science test scores each country participated in.
Our World in Data is free and accessible for everyone.
Help us do this work by making a donation.