For longitudinal studies such as Young Lives, getting comparable measures of children’s cognitive abilities over time is essential for identifying individual, family, school or contextual variables that affect their development. Few longitudinal studies that follow birth/age cohorts have comparable cognitive measures over time; of those that are available, most are from developed countries and there are almost none from developing countries. For example, studies such as The National Education Longitudinal Study (NELS), the Early Childhood Longitudinal Study – Kindergarten (ECLS-K), Education Longitudinal Study (ELS), and the Rochester Longitudinal Study in the United States have achievement measures (maths and reading comprehension) that are comparable across waves. To help fill this gap, this technical note outlines the statistical procedures that Young Lives has followed to achieve comparable measures across rounds and age cohorts in Ethiopia, India, Peru and Vietnam.