The substitutability between class size and instructional hours and its impact on academic achievement

The past few years there is an ongoing discussion regarding large classes in Israel. This study examines the relationship between class size, instruction hours, and academic achievements from an international perspective. The study has found no significant statistical correlation between these variables. The study’s authors, Nachum Blass, Prof. Benjamin Bental, and Michael Debowy, suggest that to see a major improvement in academic achievements, there is a need to make significant changes in class size and instructional hours alongside pedagogical processes, since there is no evidence that small changes of these factors alone impact achievements.

The education system budget, which is the second largest budget following defense and is composed primarily of teachers’ salaries, has a direct relationship to class size (which dictates the number of classes and the number of teachers) and the number of instructional hours per class. Every class (in primary and middle school) is allocated an equal basic budget, between 29 and 32 hours, with additional hours allocated for each student over the 20th student. The majority of studies have shown that larger classes result in somewhat lower achievements, particularly among younger students and students from weaker socioeconomic backgrounds. Adding or subtracting instructional hours has a limited impact on academic achievements as well. That is, from a pedagogical perspective, there is some justification for reducing the number of students in a class and the number of teaching hours simultaneously. Taub Center researchers point to an option where the added costs of reducing the number of students in a class will be totally or partly eliminated by a reduction in the number of instructional hours per class. Reducing the number of instructional hours can balance the budget costs of lowering class size, and lowering class size can balance the price of reducing instructional hours in terms of student achievements.

The study examines the relationship between class size and instructional hours and academic achievements to determine if it is possible to reduce instructional hours while simultaneously reducing class size without harming student academic achievement thus eliminating the need for additional budget. To determine this, the researchers relied on results of the international TIMSS (primary school) and PISA (post-primary) exams for the years 1999–2019 as well as on data regarding class size and instruction hours in 31 countries participating in these international exams.

With regard to the relation between average class size and student scores in the various countries, no clear relationship was found between the variables, largely because the differences between countries — in terms of class size and exam scores — were far greater than the changes within the countries over time. So, for example, in Japan and South Korea class sizes are large and test scores are high, whereas in Mexico and Israel, class sizes are large and exam scores are low. Similarly, there was no clear indication of a relationship between instructional hours and student achievement.

Ultimately, no statistically significant correlation between class size and instructional hours and student academic achievement was found. Even substantial changes in the number of students per class when accompanied by a change in instructional hours does not show a clear change in academic achievements. What is more, even in countries where both factors were changed or where one factor was changed and was accompanied by a change in achievements, the direction of the change was not clear cut. That is, there were countries that lowered class size and instructional hours and student achievements improved, and there were countries with the opposite results from such a change. It thus appears that a significant and reliable impact over time on student achievements will only be accomplished when accompanied by the appropriate pedagogical changes.