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In 2015, TIMSS was administered in 49 IEA member countries and 6 other education systems at grade 4, and in 38 IEA member countries and 6 other education systems at grade 8.1

IEA member countries include both "countries," which are complete, independent political entities and "other education systems," or non-national entities (e.g., the Flemish community of Belgium, England). Non-national entities that are not IEA member countries (i.e., Florida, Abu Dhabi) are designated as "benchmarking participants." "Other education systems" are indicated in the tables and figures with the three-letter international abbreviation for their nation following their name (e.g., England-GBR, Florida-USA). (For more information on terminology, see the Textbox below.) For convenience, the generic term "education systems" is used when summarizing across results.

For a number of education systems, changes in achievement can be documented over the last 20 years, from 1995 to 2015. For those that began participating in TIMSS data collections after 1995, changes can be documented over a shorter period of time. The TIMSS participation table shows the education systems that participated in TIMSS 2015 as well as their participation status in the earlier TIMSS data collections. The TIMSS 4th-grade assessment was implemented in five TIMSS cycles (1995, 2003, 2007, 2011, and 2015), while the 8th-grade assessment was implemented in all six cycles (including 1999).

Countries or Education Systems?

The international bodies that coordinate international assessments vary in the labels they apply to participating entities. For example, the IEA, which coordinates TIMSS and the Progress in International Reading Literacy Study (PIRLS), differentiates between IEA members, which the IEA refers to as "countries" in all cases, and "benchmarking participants." IEA members include countries such as the United States and Japan, as well as subnational entities, such as England and Northern Ireland (which are both part of the United Kingdom); the Flemish community of Belgium; Hong Kong, which is a Special Administrative Region of China; and Chinese Taipei, which is also a part of China. IEA benchmarking participants—typically all subnational entities—included the U.S. state of Florida and sub-national entities from five other countries in 2015. The Organization for Economic Cooperation and Development (OECD), which coordinates the Program for International Student Assessment (PISA), differentiates between OECD member countries and all other participating entities (called "partner countries" or "partner economies"), which include countries and subnational entities. In PISA, the United Kingdom and Belgium are reported as whole countries.

The TIMSS 2015 Results, which are primarily adapted from the TIMSS 2015 international reports, follow the IEA TIMSS convention of placing members and nonmembers in separate parts of the tables and figures in order to facilitate readers moving between the international reports and the U.S. national reporting. When summarizing across countries and other education systems, however, NCES uses the term “education systems” for convenience. (Note also that the 2015 Highlights Report Results does not follow the convention of placing benchmarking participants in separate parts of tables and figures.)

1 This count of countries and other education systems differs from the totals in the international reports because it excludes those countries that gave the assessments only off-grade (e.g., to 6th or 9th grade students only), which the international reports include. This count of countries and other education systems thus matches the count in the TIMSS 2015 Results and in the TIMSS participation table minus the off-grade participants.