Understanding causal influences on educational achievement through analysis of within-country differences over time

When the International Association for the Evaluation of Educational Achievement (IEA) was established in 1959 the basic idea was to use comparative analysis of country differences in achievement to take advantage of the world as an educational laboratory. A large number of comparative studies involving substantial numbers of countries has since then been conducted and much has indeed been learned. However, it has also been learned that causal inference from cross-sectional comparative data is a weak method for gaining knowledge about which factors are conducive to educational achievement, because of the impossibility to control for the large number of differences between countries. During the 1990s a new generation of IEA studies was launched. These studies (e. g. TIMSS) were designed to give information about within-country trends of achievement in addition to information about between-country differences. The paper proposes that analysis of within-country differences over time is a powerful method of finding out which educational factors are related to achievement, and particularly so when the analysis involves several countries. This suggestion is illustrated through analyses of data from the TIMSS study of mathematics achievement in 1995 and 2003 for grades 4 and 8, investigating effects of age and class size on achievement.