Walt Gardner puts PISA in perspective and explains why China's scores are questionable

http://blogs.edweek.org/edweek/walt_gardners_reality_check/2010/12/the_astonishing_illiteracy_about_pisa.html

PISA measures learning that has taken place since birth, but not necessarily what students have learned during their previous year in school. (PISA has been given every three years since 2000 to 15-year-olds.) As a result, it's extremely difficult to disentangle school effects from non-school effects. Although this distinction is crucial, it is given short shrift by the media in their reportage and commentary.

It's also vital to determine if a true sample of students from each country is being tested. It's here that China's results are highly suspect. About 5,100 students only from Shanghai were chosen. But Shanghai is hardly representative of China because it is an industrialized center with scores of modern universities. In contrast, the U.S. selected students from both public and private schools across the nation.