There are surprisingly few high-quality studies exploring the effects of adult/child ratio and group size in early childhood education and care (ECEC) using a methodologically suitable study design. Based on the available evidence, it is not possible to draw any definitive conclusions regarding the impact of adult/child ratio and group size on children in ECEC. However, the results of a meta-analysis tentatively suggest that fewer children per adult and smaller group sizes do increase process quality – defined as more positive adult/child and child/child interactions, less coercive and controlling adult interference, and less aggressive and more prosocial child behaviour.

Background: Worldwide, a large number of infants, toddlers, and preschoolers are enroled in formal non-parental early childhood education or care (ECEC). Theoretically, lower adult/child ratios (fewer children per adult) and smaller group sizes are hypothesised to be associated with positive child outcomes in ECEC. A lower adult/child ratio and a smaller group size may increase both the extent and quality of adult/child interactions during the day.

Objectives: The objective of this review is to synthesise data from studies to assess the impact of adult/child ratio and group size in ECEC on measures of process characteristics of quality of care and on child outcomes.

Authors' conclusions: The main finding of the present review is that there are surprisingly few quantitative studies exploring the effects of changes to adult/child ratio and group size in ECEC on measures of process quality and on child outcomes. The overall quality of the included studies was low, and only two randomised studies were used in the meta-analysis. The risk of bias in the majority of included studies was high, also in studies used in the meta-analysis. Due to the limited number of studies that could be used in the data synthesis, we were unable to explore the effects of adult/child ratio and group size separately. No study that examined the effects of changes of the adult/child-ratio and/or group size on socio-emotional child outcomes could be included in the meta-analysis. No high quality study examined the effects of large changes in adult/child ratio and group size on measures of process quality, or explored effects for children younger than 2 years. We included few studies (3) in the meta-analysis that investigated measures of language and literacy and results for these outcomes were inconclusive. In one specification, we found a small statistically significant effect on process quality, suggesting that fewer children per adult and smaller group sizes do increase the process quality in ECEC. Caution regarding the interpretation must be exerted due to the heterogeneity of the study designs, the limited number of studies, and the generally high risk of bias within the included studies. Results of the present review have implications for both research and practice. First, findings from the present review tentatively support the theoretical hypothesis that lower adult/child ratios (fewer children per adult) and smaller group sizes beneficially influence process quality in ECEC. This hypothesis is reflected in the existence of standards and regulation on the minimum requirements regarding adult/child ratios and maximum group size in ECEC. However, the research literature to date provides little guidance on what the appropriate adult/child ratios and group sizes are. Second, findings from the present review may be seen as a testimony to the urgent need for more contemporary high-quality research exploring the effects of changes in adult/child ratio and group size in ECEC on measures of process quality and child developmental and socio-emotional outcomes.