Linking the first, second and third IEA science studies

International comparative assessments are crucial for examining country-level educational changes over time, but requires a comparable achievement scale. This study develops a unified science achievement scale that links the First and Second International Science Studies with the Trends in International Mathematics and Science Study for upper primary school students. The science measures in each study are analyzed with respect to construct consistency, inferences, population differences, and test conditions. Analyses showed that, despite some evolution in frameworks and a transition from age-based to grade-based definitions, the core constructs remain consistent, enabling similar inferences. By identifying common items among the three studies, we rigorously assessed measurement invariance using item response modeling, with partial invariance models supporting reliable long-term comparisons. We applied latent regression models using contextual student data, creating imputed scale scores for each student. These results enable vast further research to investigate long-term inequalities in educational outcomes. • Analyzed degrees of similarity for consistency across the FISS, SISS, and TIMSS. • Conducted measurement invariance analysis of the FISS, SISS, and TIMSS. • Generated plausible values for the FISS and SISS linked to TIMSS 1995 scores. • Provided trends in science achievement scores across FISS, SISS and TIMSS 1995. • Findings support research on long-term inequalities in educational outcomes.