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.
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