Assessing the Effects of Instructional Time on Student Achievement: Empirical Evidence from the TIMSS Assessments (1995-2007)

Abstract

Policymakers at the highest level have recently called for broad increases in instructional time to improve student achievement in the United States. Nevertheless, due to disparities in methodologies and focuses, previous empirical research on the effectiveness of expanding instructional time has produced largely mixed and inconclusive results. This paper investigates the effects of changes in instructional hours per day and days per year (and, by extension, the distribution of instructional time) on eighth-grade student achievement using the international TIMSS Assessments. I merged data from four quadrennial testing cycles (between 1995 and 2007) – covering more than 800,000 student observations across seventy countries – to exploit the tremendous variation in instructional time across countries and years. This study finds that the relationship between student achievement and instructional time follows a nonlinear, concave specification. In particular, I find that achievement can be maximized at particular levels of instructional time – 181 days per year and 7 hours per day. Given that the levels of instructional time for the average school in the United States appear to be near this optimum, the marginal effect of an extra day in the U.S. is statistically insignificant, while an additional hour per day increases achievement by only 0.011 standard deviations. I also find that these effects are heterogeneous for different cohorts of students – namely, underprivileged students derive greater benefits from increased instructional time than those who are more privileged. Thus, this study provides evidence that an expansion of instructional time cannot be expected to automatically increase student achievement, and extending the length of the school day appears to be more effective than extending the length of the school year. Policymakers would also be well advised to consider both the context and costs surrounding an increase in instructional time.