This book offers a comprehensive analysis of the science underpinning all IEA’s ILSAs. The content bridges gaps in the general knowledge of consumers of reported outcomes of IEA studies, providing readers with the understanding necessary to properly interpret the results as well as critical insight into the way in which IEA, through its expressed methodologies, has addressed concerns related to quality, with particular reference to issues of reliability and validity. To a large extent, the chapters in the publication reflect the chronology of the development and execution of a large-scale assessment and provide a thorough overview of some of the key challenges that confront developers of IEA assessments and the strategies adopted to address them. While the issues and strategies outlined may also be common to other ILSAs, the focus of this volume is on the work of IEA; the chapters present examples that reflect some of the problems common to most ILSAs, but include those that are unique to particular IEA studies.