How much do 15-year-olds learn over one year of schooling?

In 2020 and 2021, schooling, like many other aspects of life, has been heavily affected by the ongoing coronavirus pandemic. During periods of school closure, education systems and schools have often been quick to organise remote support for home-based learning. But several observers have questioned the effectiveness of these schooling surrogates, either in general or for particular types of students. Initial data from national assessments confirm that the results of many students who experienced school closures (particularly those from disadvantaged backgrounds) lag behind those of similar students in previous school years. Learning losses and gains are often compared to the typical learning progression observed in normal times over a year of schooling. This paper shows how international data can be used to investigate learning losses and gains over a school year.