In addition to the following questions about TIMSS, more FAQs about international assessments are available at: http://nces.ed.gov/surveys/international/faqs.asp.
The TIMSS 2011 results at 8th grade, the grade closest to the age of the PISA students, showed U.S. average scores higher than the TIMSS scale average in both mathematics and science. In PISA 2009, the average scores of U.S. 15-year-old students were below (in mathematics) or not measurably different (in science) from the OECD average—the average score of students in the 34 Organization for Economic Cooperation and Development countries. How do we reconcile the apparent differences?
The results from TIMSS and PISA are difficult to compare because the assessments are so different in at least three key ways that could influence results. First, TIMSS assesses 8th- and 4th-graders, while PISA is an assessment of 15-year-old students, regardless of grade level. (In the United States, PISA data collection occurs in the autumn, when most 15-year-olds are in 10th grade.) So, the grade levels of students in PISA and TIMSS differ. Second, the knowledge and skills measured in the two assessments differ. TIMSS is intended to measure how well students have learned the mathematics and science curricula in participating countries, whereas PISA is focused on application of knowledge to "real-world" situations. Third, the participating countries in the two assessments differ. Both assessments cover much of the world, but they do not overlap neatly. Only 25 of the 42 participating education systems in TIMSS 2011 at the 8th-grade level participated in the PISA 2009 assessment of 15-year-olds. Both assessments include key economic competitors and partners, but the overall makeups of the countries participating in the two assessments differ markedly. Thus, the "averages" used by the two assessments are in no way comparable, and the "rankings" often reported in media coverage of these two assessments are based on completely different sets of countries.
To learn more about how the TIMSS assessment differs from PISA as well as NAEP, see the following paper: Comparing TIMSS with NAEP and PISA in Mathematics and Science (2007) (281 KB)