Trends in these variables between 2003 and 2012 are examined when comparable data is available, and case studies, examining the policy reforms adopted by countries that have improved in PISA, are highlighted throughout the volume. It provides the rationale for assessing problem-solving skills and describes performance within and across countries. In addition, the volume highlights the relative strengths and weaknesses of each school system and examines how they are related to individual student characteristics, such as gender, immigrant background and socio-economic status.