Comparisons were made between those scoring at the "advanced" and "proficient" levels in mathematics on the NAEP with the corresponding performance on the PISA. Overall, 30 nations had higher percentages than the U.S. of students at the "advanced" level of mathematics. The only OECD countries with worse results were Portugal, Greece, Turkey, and Mexico. Six percent of U.S. students were "advanced" in mathematics compared to 28 percent in Taiwan. The highest ranked state in the U.S. (Massachusetts) was just 15th in the world if it was compared with the nations participating in the PISA. 31 nations had higher percentages of "proficient" students than the U.S. Massachusetts was again the best U.S. state, but it ranked just ninth in the world if compared with the nations participating in the PISA. Comparisons with results for the Trends in International Mathematics and Science Study (TIMSS) appear to give different results—suggesting that the U.S. states actually do better in world rankings. This can likely be traced to the different material being covered and the United States teaching mathematics in a style less harmonious with the "Realistic Mathematics Education" which forms the basis of the exam. Countries that commonly use this teaching method score higher on PISA, and less highly on TIMSS and other assessments. Stephen Krassen, professor emeritus at the University of Southern California, and Mel Riddile of the NASSP attributed the relatively low performance of students in the United States to the country's high rate of child poverty, which exceeds that of other OECD countries. However, individual US schools with poverty rates comparable to Finland's (below 10%), as measured by reduced-price school lunch participation, outperform Finland; and US schools in the 10–24% reduced-price lunch range are not far behind. Reduced school lunch participation is the only available intra-poverty indicator for US schoolchildren. In the United States, schools in locations in which less than 10% of the students qualified for free or reduced-price lunch averaged PISA scores of 551 (higher than any other OECD country). This can be compared with the other OECD countries (which have tabled figures on children living in relative poverty):  In 2013 Martin Carnoy of the Stanford University Graduate School of Education and Richard Rothstein of the Economic Policy Institute released a report, "What do international tests really show about U.S. student performance?", analyzing the 2009 PISA data base.