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Group Size in Early Childhood Classrooms
June 2020


What does the research say about how group size and child-adult ratios in early childhood classrooms relate to child outcomes?

Ask A REL Response

Thank you for your request to our Regional Educational Laboratory (REL) Reference Desk. Ask A REL is a collaborative reference desk service provided by the 10 RELs that, by design, functions much in the same way as a technical reference library. Ask A REL provides references, referrals, and brief responses in the form of citations in response to questions about available education research.

Following an established REL Northwest research protocol, we conducted a search for evidence- based research. The sources included ERIC and other federally funded databases and organizations, research institutions, academic research databases, Google Scholar, and general Internet search engines. For more details, please see the methods section at the end of this document.

The research team has not evaluated the quality of the references and resources provided in this response; we offer them only for your reference. The search included the most commonly used research databases and search engines to produce the references presented here. References are listed in alphabetical order, not necessarily in order of relevance. The research references are not necessarily comprehensive and other relevant research references may exist. In addition to evidence-based, peer-reviewed research references, we have also included other resources that you may find useful. We provide only publicly available resources, unless there is a lack of such resources or an article is considered seminal in the topic area.


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.

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."

Balestra, S., & Backes-Gellner, U. (2017). Heterogeneous effects of pupil-to-teacher ratio policies-A look at class size reduction and teacher aide. Economics of Education Working Paper Series, (102). Retrieved from

From the Abstract:
"This paper investigates the effect of two pupil-to-teacher ratio policies on test scores for children with different achievement levels. Using data from a large randomized experiment in early childhood, we estimate unconditional quantile treatment effects of small class and teacher aide, as compared to regular classes. For the small class intervention, results show that pupils in the middle of the achievement distribution profit the most from being assigned to a smaller class, whereas pupils at the bottom or at the top of the achievement distribution experience almost no gain in test scores. For the teacher aide intervention, the analysis reveals positive and significant effects for students at the bottom of the achievement distribution, an effect stronger for boys and disadvantaged pupils. The findings suggest that the average effects reported in traditional empirical studies on pupil-to-teacher ratio interventions provide an incomplete characterization of the impact on the achievement distribution, thus constituting a weak guide for policymakers."

Bratsch-Hines, M. E., Burchinal, M., Peisner-Feinberg, E., & Franco, X. (2019). Frequency of instructional practices in rural prekindergarten classrooms and associations with child language and literacy skills. Early Childhood Research Quarterly, 47(2019), 74-88. Retrieved from

From the Abstract:
"Although publicly-funded prekindergarten (pre-k) programs have been designed to promote children's school readiness, programs have tended to support early literacy skills to a greater degree than early language skills. Given the importance of both language and literacy skills for children's reading acquisition and academic achievement, the present study sought to understand whether different pre-k classroom instructional practices were related to gains in language and/or literacy skills. Teacher-child language exchanges, children's engagement in domain-specific learning activities, and the use of different types of activity settings were examined in 63 pre-k classrooms for 455 children living in six rural counties in the Southeastern United States. Hierarchical linear models showed that gains in expressive language were positively associated with teacher-child language exchanges and negatively associated with large-group activities. Gains in phonemic awareness and initial-sound knowledge were positively related to sound-focused activities and small-group settings. Gains in reading decoding skills were also positively associated with small-group settings. Implications for research, teacher practice, and professional development are discussed."

Clarke, B., Doabler, C. T., Kosty, D., Kurtz Nelson, E., Smolkowski, K., Fien, H., & Baker, S. K. (2017). Testing the efficacy of a kindergarten mathematics intervention by small group size. AERA Open, 3(2), 1-16. Retrieved from

From the Abstract:
"This study used a randomized controlled trial design to investigate the ROOTS curriculum, a 50-lesson kindergarten mathematics intervention. Ten ROOTS-eligible students per classroom (n = 60) were randomly assigned to one of three conditions: a ROOTS five-student group, a ROOTS two-student group, and a no-treatment control group. Two primary research questions were investigated as part of this study: What was the overall impact of the treatment (the ROOTS intervention) as compared with the control (business as usual)? Was there a differential impact on student outcomes between the two treatment conditions (two- vs. five-student group)? Initial analyses for the first research question indicated a significant impact on three outcomes and positive but nonsignificant impacts on three additional measures. Results for the second research question, comparing the two- and five-student groups, indicated negligible and nonsignificant differences. Implications for practice are discussed."

Neuman, S. B., & Kaefer, T. (2013). Enhancing the intensity of vocabulary instruction for preschoolers at risk: The effects of group size on word knowledge and conceptual development. Elementary School Journal, 113(4), 589-608. Retrieved from

From the Abstract:
"This study was designed to experimentally examine how supplemental vocabulary instruction provided in either whole-group or small-group settings influences low-income preschoolers' word knowledge and conceptual development. Using a within-subject design, 108 preschool children from 12 Head Start classrooms participated in an 8-week intervention, which included four topics of targeted vocabulary instruction counterbalanced in either a whole-group or small-group configuration. Pre- and posttest measures examined children's outcomes in word learning and in conceptual and categorical knowledge. Our results indicated that group size did not appear to be a powerful mechanism for intensifying instruction. Although children gained significantly in word knowledge, concepts, and categories, they did so regardless of whether they were in small or whole groups. Implications for these findings, as well as limitations of the research and directions for future research, are discussed."

Perlman, M., Fletcher, B., Falenchuk, O., Brunsek, A., McMullen, E., & Shah, P. S. (2017). Child-staff ratios in early childhood education and care settings and child outcomes: A systematic review and meta-analysis. PloS one, 12(1), 1-24. Retrieved from

From the Abstract:
"Child-staff ratios are a key quality indicator in early childhood education and care (ECEC) programs. Better ratios are believed to improve child outcomes by increasing opportunities for individual interactions and educational instruction from staff. The purpose of this systematic review, and where possible, meta-analysis, was to evaluate the association between child-staff ratios in preschool ECEC programs and children's outcomes. Searches of Medline, PsycINFO, ERIC, websites of large datasets and reference sections of all retrieved articles were conducted up to July 3, 2015. Cross-sectional or longitudinal studies that evaluated the relationship between child-staff ratios in ECEC classrooms serving preschool aged children and child outcomes were independently identified by two reviewers. Data were independently extracted from included studies by two raters and differences between raters were resolved by consensus. Searches revealed 29 eligible studies (31 samples). Child-staff ratios ranged from 5 to 14.5 preschool-aged children per adult with a mean of 8.65. All 29 studies were included in the systematic review. However, the only meta-analysis that could be conducted was based on three studies that explored associations between ratios and children's receptive language. Results of this meta-analysis were not significant. Results of the qualitative systematic review revealed few significant relationships between child-staff ratios and child outcomes construed broadly. Thus, the available literature reveal few, if any, relationships between child-staff ratios in preschool ECEC programs and children's developmental outcomes. Substantial heterogeneity in the assessment of ratios, outcomes measured, and statistics used to capture associations limited quantitative synthesis. Other methodological limitations of the research integrated in this synthesis are discussed."

Williams, P., Sheridan, S., Harju-Luukkainen, H., & Pramling Samuelsson, I. (2015). Does group size matter in preschool teacher's work? The skills teachers emphasise for children in preschool groups of different size. Journal of Early Childhood Education Research, 4(2), 93-108. Retrieved from

From the Abstract:
"In recent years there has been a debate in the Nordic countries about group size in preschool and how it affects preschool teachers' work and children's wellbeing and learning. The aim of this article is to analyse and discuss how preschool teachers' working with child groups of different sizes view the conditions for children's learning and development related to different abilities and skills. The survey data consists of preschool teachers' (N=698) responses to a questionnaire. The results indicated no statistically significant differences between preschool teachers' views and emphases in groups of different sizes as regards the type of abilities in children's learning. In all group categories, preschool teachers seem to emphasise a similar set of characteristics and social skills in children's personal development; to be collaborative, to have respect, to have empathy and a good self-esteem or understanding of oneself."

Other References

Achilles, C. M. (2012). Class-size policy: The STAR experiment and related class-size studies. NCPEA Policy Brief, 1(2), 1-9.

From the Abstract:
"This brief summarizes findings on class size from over 25 years of work on the Tennessee Student Teacher Achievement Ratio (STAR) randomized, longitudinal experiment, and other Class-Size Reduction (CSR) studies throughout the United States, Australia, Hong Kong, Sweden, Great Britain, and elsewhere. The brief concludes with recommendations. The STAR research shows that small classes (15-17 pupils) in kindergarten through third grade (K-3) provide short- and long-term benefits for students, teachers, and society at large. Although all students benefit; poor, minority, and male students reap extra benefits in terms of improved test outcomes, school engagement, and reduced grade retention and dropout rates. Differing formulas for counting students and teachers are a major impediment to understanding and using small classes correctly: a pupil-teacher ratio (PTR) is a division problem, class size is an addition problem. The two are not the same, and thus PTR data cannot be used as a substitute for actual class-size data."


Keywords and Search Strings: The following keywords, subject headings, and search strings were used to search reference databases and other sources: Class size, Group size, Ratio, Early childhood, Preschool

Databases and Resources: We searched ERIC for relevant resources. ERIC is a free online library of more than 1.6 million citations of education research sponsored by the Institute of Education Sciences (IES). Additionally, we searched Google Scholar and EBSCO databases (Academic Search Premier, Education Research Complete, and Professional Development Collection).

Reference Search and Selection Criteria

When we were searching and reviewing resources, we considered the following criteria:

Date of publications: This search and review included references and resources published in the last 10 years.

Search priorities of reference sources: Search priority was given to study reports, briefs, and other documents that are published and/or reviewed by IES and other federal or federally funded organizations, as well as academic databases, including ERIC, EBSCO databases, and Google Scholar.

Methodology: The following methodological priorities/considerations were given in the review and selection of the references:

  • Study types: randomized control trials, quasi experiments, surveys, descriptive data analyses, literature reviews, and policy briefs, generally in this order
  • Target population and samples: representativeness of the target population, sample size, and whether participants volunteered or were randomly selected
  • Study duration
  • Limitations and generalizability of the findings and conclusions

This memorandum is one in a series of quick-turnaround responses to specific questions posed by stakeholders in Alaska, Idaho, Montana, Oregon, and Washington, which is served by the Regional Educational Laboratory (REL) Northwest. It was prepared under Contract ED-IES-17-C-0009 by REL Northwest, administered by Education Northwest. The content does not necessarily reflect the views or policies of IES or the U.S. Department of Education, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.