This paper investigates the heterogeneous effects of changes in instructional time on student achievement due to differences in instructional quality, using student test scores and data on teachers and schools from the Trends in International Mathematics and Science Study (TIMSS), as well as a natural experiment caused by a 2002 Japanese curriculum standards revision that reduced instruction time for math but not for science for second-year junior high school (grade 8) students. The random matching of teachers and students in Japan and our application of the time dimension of panel data to two subjects, mathematics and science, allow us to control for unobserved heterogeneities such as individual fixed effects and school fixed effects. We confirm that instructional time is more effective in combination with higher-quality teachers, as measured by common indicators such as experience, schooling, and academic major. We also find that these effects are larger for students whose socioeconomic status (SES) is lower.
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