The OECD’s Program for International Student Assessment (PISA), which tests 15-year-old students’ abilities in math, reading and science, also measures the relationship between students’ socioeconomic backgrounds and their performance. In 2018, the score difference in reading between the most socioeconomically advantaged students and the most socioeconomically disadvantaged students in the United States was much larger than that seen in top-performing education systems, especially Hong Kong and Estonia. Whereas students from the bottom quarter of the PISA index of economic, social and cultural status (ESCS) in the U.S. performed about 99 points behind those from the top quarter, that gap was only 59 points in Hong Kong and 61 points in Estonia. PISA’s index of ESCS takes several variables into account, including parents’ education and occupations as well as the number of books and other educational resources available in students’ homes. For more on how PISA tests student reading performance, see the PISA 2018 Assessment and Analytical Framework.