Countries around the world are invited to participate in each assessment by the sponsoring international organization. Because they volunteer to participate, the number and range of countries (e.g., developed vs. developing) vary from assessment to assessment. Though TIMSS, PIRLS, and PISA include developed and developing countries, a larger proportion of developing countries have participated in TIMSS and PIRLS than in PISA and ALL (table 1). PISA is primarily administered in the member countries of the OECD—an intergovernmental organization of 30 industrialized countries seeking to promote trade and economic growth. ALL was conducted only among 6 countries in 2003, but additional countries collected data in 2005, and more countries plan to participate in future years

Differences in the combinations of countries that participate in the assessments can affect how various measures, such as the international average, are calculated and interpreted. For example, because national average scores in developing countries tend to be lower than those in developed countries, the international averages can vary from administration to administration, depending on which countries participate. In TIMSS and PIRLS, the international averages are calculated using results from both developing and developed countries while in PISA, they are calculated using results only from the OECD-member countries.