Student testing is increasingly popular around the world. Though it aids accountability, people have begun to wonder about the consequences of too much testing. In this video, LUDGER WOESSMANN analyses the effects on student learning of different types of testing. With PISA (the Programme for International Student Assessment) providing data on students, schools and testing regimes in 59 participating countries, the study identifies striking variations in effectiveness between different forms of testing. While the introduction of standardized comparative testing is observed to lead to significant improvements in student learning, internal school tests, whether used to measure the performance of students or teachers, are shown to have little discernible effect. As clear links have been identified between a nation’s performance in studies like PISA and its prospects for economic growth, Woessmann’s work has vital relevance for governments and other stakeholders in educational policy worldwide.