Association for Public Policy Analysis & Management

This paper examines the effects of changes in instructional time on eighth-grade student achievement through a new methodological framework that disaggregates total yearly in- structional time into separate inputs for days per year and hours per day. This study’s dataset brings together nearly 900,000 student observations across 80 countries and four quadrennial testing cycles of the international TIMSS Assessments (1995-2007). I find that student achievement can actually be maximized at particular levels of instructional time not far from the current national averages in the United States, with the length of the school day (and not the school year) serving as the primary driver of changes in outcomes. 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. While socioeconomically underprivileged students are more likely to realize gains from a longer school day (with diminishing marginal returns in fact vanishing for the poorest), these relative benefits are driven by varying responses to time spent in non-tested subjects. On the other hand, the effects of time spent in tested subjects remain homogeneous across student groups.