The Effect of Additional Time in School on Students’ Cognitive Skills

Across the country, parents and educators are debating how much time students should spend in school. In Chicago, a teacher’s strike concluded with, among other things, an agreement to extend the length of the school day for public school students. But does more time in the classroom mean smarter students?

A recent working paper from the National Bureau of Economic Research, by Magnus Carlsson, Gordon B. Dahl, and Dan-Olof Rooth, attempts to identify whether spending more time in school has an effect on students’ cognitive skills. The paper, The Effect of Schooling on Cognitive Skills, uses test data from Sweden, where almost all 18-year-old men are required to undergo cognitive testing in advance of military service. These tests include a verbal exam and a technical comprehension exam, known as “crystallized intelligence tests,” as well as spatial and logic tests, called “fluid intelligence tests.” These exams are similar to ones employed by the US military, some companies, and college entrance exams, so the authors’ findings could be relevant to American students as well.

The timing of Swedish military enlistment is random, so students taking the exams vary in how old they are and how much time they have spent in school, though they are all in the twelfth grade. This also means that when students take the test has no relationship to their family background, prior school performance, or other characteristics that might influence their performance.

After examining students’ test results and comparing differences in age and time spent in school, the researchers drew several conclusions. First, an additional 10 school days led to an increase of 1.1 percent of a standard deviation on the verbal skills test and an increase of 0.8 percent of a standard deviation on the technical test. However, students who were older, but had spent the same number of days in school as their younger peers fared no better or worse than younger students.

When extrapolated to a student spending an additional year or more in school (cumulatively, due to longer school days or years, by the time they finish high school), these results could be quite significant. Assuming results are cumulative across grades, the authors posit that extending the school year by 20 days in kindergarten could improve student performance enough to lift the US from fourteenth to fourth on international tests of academic performance.

Scores on the fluid intelligence tests were unaffected by additional school days, but did increase with age. Importantly, the researchers also found no difference in the test score increases among students of different backgrounds. Students with different levels of parental education, father’s income, and academic achievement all experienced similar effects from the additional school days.

A second finding from the study was that, because older test takers tended to have higher cognitive scores, the researchers conclude that students’ cognitive skills are still developing at age 18. The paper states that, for both crystallized and fluid intelligence, “even as late as age 18, these types of cognitive skills are not fully determined, and therefore cannot easily be compared across individuals who are different ages when they take the tests.” This finding in particular has implications for some of the standardized tests used frequently in the US.

The authors also make an important distinction between the crystallized and fluid tests. It is expected that crystallized intelligence would increase with time spent in school, as it measures concepts that can be taught and learned, while fluid intelligence measures abstract skills and critical thinking—and is apparently not affected by additional time spent in school.

Results from this study suggest that more time in school can lead to increases in scores on verbal and technical comprehension exams, while simply being older contributes to increases in scores on tests of abstract skills and critical thinking. Policymakers may turn to the results of this study when considering if, and how, to alter public school calendars.

Feature Photo: cc/chrisvick'
Alex Usher
Alex Usher is the 2013-14 Executive Editor for Research Briefs and is an MPP student at the Harris School of Public Policy. She is interested in education and urban social policy.

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