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Since the early days of international large-scale assessments, an overarching aim has been to use the world as an educational laboratory so countries can learn from one another and develop educational systems further. Cross-sectional comparisons across countries as well as trend studies derive from the assumption that there are comparable groups of students in the respective samples. But neither age-based nor grade-based sampling strategies can achieve balanced samples in terms of both age and schooling. How should such differences in the sample compositions be dealt with?

A correction model for differences in the sample compositions: the degree of comparability as a function of age and schooling