In international tests administered to students in dozens of nations, U.S. fourth graders rank in the top 30% of countries for math performance. However, U.S. students plunge to the bottom 20% by the age of 15. Randi Weingarten, president of one of the largest U.S. teachers' unions, has blamed the subpar performance of older U.S. students on a lack of education funding, but average spending per U.S. public school student was 37% above the average of the tested nations. This is measured in "purchasing power parities," which allow for accurate international comparisons because they account for the cost of living in different nations. U.S. public schools spend an average of $15,621 per student per year, or $353,028 per classroom. This excludes certain expenses like state administration and unfunded pension liabilities.