What is the impact of class size on student outcomes?

Thank you for the question you submitted to our REL Reference Desk regarding the impact of class size on student outcomes. We have prepared the following memo with research references to help answer your question. For each reference, we provide an abstract, excerpt, or summary written by the study's author or publisher. The references are selected from the most commonly used research resources and may not be comprehensive. References are listed in alphabetical order, not necessarily in order of relevance. Other relevant studies may exist. We have not evaluated the quality of these references, but provide them for your information only.

Research References

1. Alharbi, A. A. & Stoet, G. (2017). Achievement flourishes in larger classes: Secondary school students in most countries achieved better literacy in large classes. International Education Journal: Comparative Perspectives, 16(2), 16-32.
   Retrieved from: https://eric.ed.gov/?id=EJ1146648
   From the abstract: “There is no consensus among academics about whether children benefit from smaller classes. We analyzed the data from the 2012 Program for International Student Assessment (PISA) to test if smaller classes lead to higher performance. Advantages of using this data set are not only its size (478,120 15-year old students in 63 nations) and representativeness but also that the 2012 PISA data set, for the first time, includes the class size for each participating child. We found that, in most countries, children in smaller classes had a lower performance score in solving reading comprehension problems than those in larger classes. We further analyzed the relationship between class size and factors that can explain this paradoxical phenomenon. Although grouping of students by ability and the socioeconomic status of parents played some role in some countries, these factors cannot fully explain the effect. We finish by discussing the overlooked potential advantages of larger classes.”
2. Bettinger, E. P. & Long, B. T. (2018). Mass instruction or higher learning? The impact of college class size on student retention and graduation. Education Finance and Policy, 13(1), 97-118.
   Retrieved from: https://eric.ed.gov/?id=EJ1165328
   Request full text available at https://www.researchgate.net/publication/305920134
   From the abstract: “This paper measures the effects of collegiate class size on college retention and graduation. Class size is a perennial issue in research on primary and secondary schooling. Few researchers have focused on the causal impacts of collegiate class size, however. Whereas college students have greater choice of classes, selection problems and nonrandom sorting make it difficult to estimate causal impacts. Using unique data and exogenous variation in class size, we estimate the impacts of class size using a sample of nearly 60,000 four-year college students. Using an instrumental variables approach to control for selection bias, the results suggest an increase in collegiate class size leads to an increase in dropout rates and a reduction in on-time degree completion, but no change in long-run degree completion.”
3. Bosworth, R. (2014). Class size, class composition, and the distribution of student achievement. Education Economics, 22(2), 141-165.
   Retrieved from: https://eric.ed.gov/?id=EJ1028089
   Request full text available at https://www.researchgate.net/publication/254248669
   From the abstract: “Using richly detailed data on fourth- and fifth-grade students in the North Carolina public school system, I find evidence that students are assigned to classrooms in a non-random manner based on observable characteristics for a substantial portion of classrooms. Moreover, I find that this non-random assignment is statistically related to class size for a number of student characteristics and that failure to control for classroom composition can severely bias traditionally estimated class size effects. Teacher-fixed effects and classroom composition controls appear to be effective at addressing selection related to classroom composition. I find heterogeneity in class size effects by student characteristics--students who struggle in school appear to benefit more from class size reductions than students in the top of the achievement distribution. I find that smaller classes have smaller achievement gaps on average and that class size reductions may be relatively more effective at closing achievement gaps than raising average achievement; however, class size effects on both average achievement and achievement gaps are small.”
4. Bowne, J.B., Magnuson, K.A., Schindler, H.S., Duncan, G.J., & Yoshikawa, H. (2017). A meta-analysis of class sizes and ratios in early childhood education programs: Are thresholds of quality associated with greater impacts on cognitive, achievement, and socioemotional outcomes? Educational Evaluation and Policy Analysis, 39(3), 407-428.
   Retrieved from: https://eric.ed.gov/?id=EJ1149537
   From the abstract: “This study uses data from a comprehensive database of U.S. early childhood education program evaluations published between 1960 and 2007 to evaluate the relationship between class size, child-teacher ratio, and program effect sizes for cognitive, achievement, and socioemotional outcomes. Both class size and child-teacher ratio showed nonlinear relationships with cognitive and achievement effect sizes. For child-teacher ratios 7.5:1 and lower, the reduction of this ratio by one child per teacher predicted an effect size of 0.22 standard deviations greater. For class sizes 15 and smaller, one child fewer predicted an effect size of 0.10 standard deviations larger. No discernible relationship was found for larger class sizes and child--teacher ratios. Results were less clear for socioemotional outcomes due to a small sample.”
5. Chingos, M. M. (2012). The impact of a universal class-size reduction policy: Evidence from Florida's statewide mandate. Economics of Education Review, 31(5), 543-562.
   Retrieved from: https://eric.ed.gov/?id=EJ988626
   Request full text available at https://www.researchgate.net/publication/257105904
   From the abstract: “Class-size reduction (CSR) mandates presuppose that resources provided to reduce class size will have a larger impact on student outcomes than resources that districts can spend as they see fit. I estimate the impact of Florida's statewide CSR policy by comparing the deviations from prior achievement trends in districts that were required to reduce class size to deviations from prior trends in districts that received equivalent resources but were not required to reduce class size. I use the same comparative interrupted time series design to compare schools that were differentially affected by the policy (in terms of whether they had to reduce class size) but that did not receive equal additional resources. The results from both the district- and school-level analyses indicate that mandated CSR in Florida had little, if any, effect on student achievement.”
6. Cho, H., Glewwe, P., & Whitler, M. (2012). Do reductions in class size raise students' test scores? Evidence from population variation in Minnesota's elementary schools. Economics of Education Review, 31(3), 77-95.
   Retrieved from: https://eric.ed.gov/?id=EJ964337
   Request full text available at https://www.researchgate.net/publication/254407897
   From the abstract: “Many U.S. states and cities spend substantial funds to reduce class size, especially in elementary (primary) school. Estimating the impact of class size on learning is complicated, since children in small and large classes differ in many observed and unobserved ways. This paper uses a method of Hoxby (2000) to assess the impact of class size on the test scores of grade 3 and 5 students in Minnesota. The method exploits random variation in class size due to random variation in births in school and district catchment areas. The results show that reducing class size increases mathematics and reading test scores in Minnesota. Yet these impacts are very small; a decrease of ten students would increase test scores by only 0.04-0.05 standard deviations (of the distribution of test scores). Thus class size reductions are unlikely to lead to sizeable increases in student learning.”
7. Lin, C., Kwon, J. B., & Zhang, Y. (2019). Online self-paced high-school class size and student achievement. Educational Technology Research and Development, 67(2), 317- 336.
   Retrieved from: https://eric.ed.gov/?id=EJ1208091
   Full text available at https://www.researchgate.net/publication/326339921
   From the abstract: “In the traditional brick-and-mortar classroom, small classes are generally perceived as desirable, but the benefits associated with particular class sizes in online education have not yet received much scholarly attention. Using a dataset of 10,648 enrollment records generated during the 2013-2014 school year at a state virtual school in the Midwestern U.S., this study examined the relationship between class size and student learning outcomes. The results of hierarchical linear modeling with fractional polynomial analysis suggest a reverse-U-shaped relationship, in which increasing online class sizes had a positive impact on achievement until the number of students reached 45, but a negative one if numbers increased beyond that level. At the subject level, similar reverse- U-shaped patterns were observed in math, social science, and other subjects, but not in English, foreign languages, or science.”
8. Nandrup, A. B. (2016). Do class size effects differ across grades? Education Economics, 24(1), 83-95.
   Retrieved from: https://eric.ed.gov/?id=EJ1086779
   Full text available at http://iwaee.org/PaperValidi2014/20140214103857_140127_Do_Class_Size_Effects_Differ_Across_Grades.pdf
   From the abstract: “This paper contributes to the class size literature by analyzing whether short-run class size effects are constant across grade levels in compulsory school. Results are based on administrative data on all pupils enrolled in Danish public schools. Identification is based on a government-imposed class size cap that creates exogenous variation in class sizes. Significant (albeit modest) negative effects of class size increases are found for children at primary school levels. The effects on math achievement are statistically different across grade levels. Larger classes do not affect girls, non-Western immigrants and socioeconomically disadvantaged pupils more adversely than other pupils.”
9. Shin, Y. (2012). Do Black children benefit more from small classes? Multivariate instrumental variable estimators with ignorable missing data. Journal of Educational and Behavioral Statistics, 37(4), 543-574.
   Retrieved from: https://eric.ed.gov/?id=EJ973867
   Full text available at https://www.classsizematters.org/wp-content/uploads/2014/08/do-black-children-benefit-more-from-small-classes.pdf
   From the abstract: “Does reduced class size cause higher academic achievement for both Black and other students in reading, mathematics, listening, and word recognition skills? Do Black students benefit more than other students from reduced class size? Does the magnitude of the minority advantages vary significantly across schools? This article addresses the causal questions via analysis of experimental data from Tennessee's Student/Teacher Achievement Ratio study where students and teachers are randomly assigned to small or regular class type. Causal inference is based on a three-level multivariate simultaneous equation model (SM) where the class type as an instrumental variable (IV) and class size as an endogenous regressor interact with a Black student indicator. The randomized IV causes class size to vary which, by hypothesis, influences academic achievement overall and moderates a disparity in academic achievement between Black and other students. Within each subpopulation characterized by the ethnicity, the effect of reduced class size on academic achievement is the average causal effect. The difference in the average causal effects between the race ethnic groups yields the causal disparity in academic achievement. The SM efficiently handles ignorable missing data with a general missing pattern and is estimated by maximum likelihood. This approach extends Rubin's causal model to a three-level SM with cross-level causal interaction effects, requiring intact schools and no interference between classrooms as a modified Stable Unit Treatment Value Assumption. The results show that, for Black students, reduced class size causes higher academic achievement in the four domains each year from kindergarten to third grade, while for other students, it improves the four outcomes except for first-grade listening in kindergarten and first grade only. Evidence shows that Black students benefit more than others from reduced class size in first-, second-, and third-grade academic achievement. This article does not find evidence that the causal minority disparities are heterogeneous across schools in any given year.”
10. Taft, S. H., Kesten, K., & El-Banna, M. M. (2019). One size does not fit all: Toward an evidence-based framework for determining online course enrollment sizes in higher education. Online Learning, 23(3), 188-233.
    Retrieved from: https://eric.ed.gov/?id=EJ1228823
    From the abstract: “Class enrollment sizes for online learning in higher education, a topic of persistent interest in the academic literature, impact student learning, pedagogical strategies, school finances, and faculty workload. Yet in the research literature, class size is addressed with insufficient specificity to provide enrollment direction. Seeking guidelines for determining online class sizes, the authors conducted a qualitative research synthesis from 43 recent higher education journals, yielding 58 evidence-based articles. It is clear that no one size fits all. Findings reflect that large classes (= 40 students) are effective for foundational and factual knowledge acquisition requiring less individualized faculty-student interaction. Small classes (= 15 students) are indicated for courses intending to develop higher order thinking, mastery of complex knowledge, and student skill development. Pedagogical intent should dictate class size. Using well-established learning theories, the authors describe current understandings of online enrollments and propose an analytical framework for pedagogically driven, numerically specific class sizes. Highlights: (1) There is academic interest in online course sizes in higher education; (2) Research indicates "no one size fits all" online classes; (3) Class sizes should be based on learning level and identified pedagogical intent; (4) Large classes are appropriate for foundation-level learning; and (5) Small classes are appropriate for learning requiring higher order thinking.”
11. Wright, M. C., Bergom, I., & Bartholomew, T. (2019). Decreased class size, increased active learning? Intended and enacted teaching strategies in smaller classes. Active Learning in Higher Education, 20(1), 51-62.
    Retrieved from: https://eric.ed.gov/?id=EJ1206368
    Full text available at https://www.academia.edu/34999872
    From the abstract: “Small class size is often used as an indicator of quality in higher education, and some research suggests that instructors in smaller classes more often use activities that are learner-centered and that involve physical and mental activity on the part of learners, such as group work, simulations, and case studies. However, we have little information on how instructors change their pedagogical practice when they teach in large- versus small-class settings. In this study, we examine alignment between intended and enacted teaching strategies, or initial plans and specific ways in which instructors reported altering their teaching in the context of a university policy shift to smaller classes. Furthermore, we examine instructional challenges in this shift to call attention to professional development needs of small-class teaching and to best leverage the benefits of such activities for student performance and retention.”

Additional Organizations to Consult

• National Center for Education Research (NCER): https://ies.ed.gov/ncer/
  From the website: “The National Center for Education Research (NCER) supports rigorous research that addresses the nation's most pressing education needs, from early childhood to adult education.”
• National Education Association (NEA): http://www.nea.org/
  From the website: “The National Education Association (NEA), the nation's largest professional employee organization, is committed to advancing the cause of public education. NEA's 3 million members work at every level of education—from pre-school to university graduate programs. NEA has affiliate organizations in every state and in more than 14,000 communities across the United States.”

Methods:

Search Strings. Class size student impact OR class size student achievement OR student class size OR class size benefits OR number of students in a class OR size of class OR reducing class size OR small class size OR benefits small class size OR small class size student growth OR class size student outcomes OR class size academic achievement OR large class size OR class size challenges

Searched Databases and Resources.

• ERIC
• Academic Databases (e.g., EBSCO databases, JSTOR database, ProQuest, Google Scholar)
• Commercial search engines (e.g., Google)
• Institute of Education Sciences Resources

Reference Search and Selection Criteria. The following factors are considered when selecting references:

• Date of Publication: Priority is given to references published in the past 10 years.
• Search Priorities of Reference Sources: ERIC, other academic databases, Institute of Education Sciences Resources, and other resources including general internet searches
• Methodology: Priority is given to the most rigorous study types, such as randomized controlled trials and quasi-experimental designs, as well as to correlational designs, descriptive analyses, mixed methods and literature reviews. Other considerations include the target population and sample, including their relevance to the question, generalizability, and general quality.