The Third International Mathematics and Science Study (TIMSS) was administered to a half-million students from 41 countries in 1995. The results showed that the United States is the only country in which children do worse the longer they stay in school. Of the 21 countries participating in the 12th-grade general knowledge tests of math and science, the United States ranked 19th and 16th, respectively. Critics of international tests like TIMSS contend that American students suffer by comparison because other countries educate smaller proportions of their populations and test only "elite" students. But researchers have shown that in all 21 countries, 90 percent or more of teenagers were enrolled in school, which suggests the test is a valid comparison between American students and their foreign counterparts.