Though many factors influence the relationship between amount of instructional time and student achievement—primarily, the quality of the instruction and the students’ readiness to learn—instructional time remains a crucial component in considering students’ opportunity to learn. Instructional time was calculated using principals’ reports on the number of school days per year and the number of instructional hours per day and teachers’ reports on the weekly number of hours of science instruction, as explained in Exhibit 13.1 (see About the Scale). Exhibits 13.2 and 13.3 present principals’ and teachers’ reports about the instructional hours overall per year and hours spent on science instruction in fourth grade and eighth grade, respectively. For countries teaching science as separate subjects in the eighth grade, the instructional time included the amount of time spent on each individual science subject. Countries are ordered by the number of hours per year for science instruction.

On average, the fourth grade students across the TIMSS 2019 countries received 895 hours per year of instruction across all subjects; 73 hours, or about 8 percent of the total, were devoted to science instruction. The number of hours devoted to science instruction ranged from a high of 158 hours in the Philippines to just 34 hours in Ireland. The amount of science instructional time relative to total instructional time also varied across countries, reflecting different approaches to organizing and addressing the science curriculum. It is notable, though, that there is much less science instructional time for fourth grade students across countries compared with mathematics. As shown in Exhibit 12.2, fourth grade students had an average of 154 hours of mathematics instruction, more than twice that for science (Exhibit 13.2). As might be anticipated, within-country of estimates instructional time can vary somewhat from the levels of instructional time established by policy.

The eighth grade students across the TIMSS 2019 countries received an average of 1,023 hours of instruction across all subjects; 137 hours, or about 13 percent of the total, were devoted to science instruction. The number of hours for science instruction ranged from 243 in Lebanon, where science is taught as separate subjects, to 70 in Italy. In nearly all of the countries that participated in TIMSS at the fourth and eighth grades, the number of hours devoted to science instruction increased between fourth and eighth grades—sometimes by three or more times the average hours in fourth grade—reflecting the increased emphasis on science in the curriculum by the eighth grade.

Exhibit 13.3 also includes, for countries teaching separate science subjects, the average number of hours for biology, chemistry, physics, and Earth science. In this subset of countries, students had an estimated 181 hours of science instruction. On average, the highest number of annual hours were devoted to instruction in physics (52 hours), followed by chemistry (51 hours), biology (45 hours), and Earth science (40 hours).