Schools often have to decide between extending the length of the school year or the school day. This paper examines the effects of changes in the distribution of instructional time on eighth-grade student achievement through a methodological framework that disaggregates total yearly instructional time into separate inputs for days per year and hours per day. This study's dataset brings together nearly 900,000 student observations across eighty countries and four quadrennial testing cycles of the Trends in International Mathematics and Science Study (TIMSS) Assessments (1995–2007). I find that the positive effects of instructional time on student achievement are driven largely by the length of the school day and not by the length of the school year, with diminishing marginal returns to the former. Socioeconomically underprivileged students are most likely to realize gains from a longer school day. Furthermore, isolating the amount of instructional time spent on TIMSS-tested subjects from the rest of the school day reveals spillover effects from time spent in non-tested subjects that are especially meaningful for underprivileged students. In contrast, the effects of time spent in tested subjects are more homogeneous across student groups.

This research was done at the U.S. Census Bureau by researchers with Special Sworn Status, and the results have been through disclosure review to protect individual information. Any opinions and conclusions expressed here are those of the authors and do not necessarily represent the views of the U.S. Census Bureau or the National Bureau of Economic Research. We would like to thank the Alfred P. Sloan, Russell Sage, and Charles Koch Foundations for their generous support, Lawrence Kahn, Carla Medalia, and Ed Olsen for constructive comments, and Victoria Mooers for excellent research assistance. NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

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