Eric A. Hanushek and Ludger Woessmann (http://ssrn.com/abstract=960379)
The figure shows, by country, the share of students that scored very low (< 400 rough PISA equivalent, "scientifically and mathematically illiterate") or very high (> 600) on cognitive tests administered over the last 40 years. The results give a good indication of the quality of human capital in the country's workforce. Click for larger version.
From the paper:
...To create our measure of quality of education employed in this study, we use a simple average of the transformed mathematics and science scores over all the available international tests in which a country participated, combining data from up to nine international testing occasions and thirty individual test point observations. This procedure of averaging performance over a forty year period is meant to proxy the educational performance of the whole labor force, because the basic objective is not to measure the quality of students but to obtain an index of the quality of the workers in a country.
If the quality of schools and skills of graduates are constant over time, this averaging is appropriate and uses the available information to obtain the most reliable estimate of quality. If on the other hand there is changing performance, this averaging will introduce measurement error of varying degrees. [i.e., younger workers in developing countries probably have better skills than indicated in the data.]
More PISA fun.