If you read the article, you will see that they used the National Assessment of Educational Progress, which is a nationwide standardized test. As for how they

If you read the article, you will see that they used the National Assessment of Educational Progress, which is a nationwide standardized test. As for how they can compare the NAEP results with PISA results...well...that's also in the article (http://educationnext.org/files/ednext_XIV_4_peterson_method.jpg http://educationnext.org/files/ednext_XIV_4_peterson_method....).