The strength of the evidence on the effects of child-to-caregiver ratios on child care quality, safety, and outcomes for infants and toddlers is limited by methodology (the majority of studies use observational and correlational study designs) and a focus on older age groups (preschoolers). Most research uses natural variation in ratios across child care settings and classrooms to examine the association between ratios and various outcomes rather than evaluate the impacts of specific state policies regarding ratios or group size. Experimental studies of ratios may not be feasible because of the practical difficulties and ethical challenges associated with randomly assigning children to classrooms with better or worse child-staff ratios.
The research discussed here meets our standards of evidence for being methodologically strong and allowing for causal inference, unless otherwise noted. Each strong causal study reviewed has been assigned a letter, and a complete list of causal studies can be found at the end of this review, along with more details about our standards of evidence and review method. The findings from each strong causal study reviewed align with one of our eight policy goals from Table 4. The Evidence of Effectiveness table below displays the findings associated with lower child care ratios or group size (beneficial, null,ii or detrimental) for each of the strong studies (A and B) in the causal studies reference list, as well as our conclusions about the overall impact on each studied policy goal. The assessment of the overall impact for each studied policy goal weighs the timing of publication and relative strength of each study, as well as the size and direction of all measured indicators.
Table 5: Evidence of Effectiveness for Child Care Ratios and Group Size by Policy Goal
|Policy Goal||Indicator||Beneficial Impacts||Null Impacts||Detrimental Impacts||Overall Impact on Goal|
|Optimal Child Health and Development||Child Cognitive Development||A||Mixed|
|Child Social-Emotional Health||A|
|Frequency of Illness (Respiratory Infections)||B|
Optimal Child Health and Development
Most research on the impact of child-to-caregiver ratios in formal child care centers examines how ratios contribute to overall process qualityiii or the quality of caregiving and instruction (e.g., frequency of child-caregiver interactions, positive regard shown by caregiver).11 Fewer studies make the connection between ratios and specific cognitive or developmental outcomes for children. The studies that do examine cognitive and developmental outcomes tend to have less rigorous designs, including small sample sizes,12 or focus on preschool-aged children rather than children under age 3.13,30 A 1979 study, the National Day Care Study (NDCS), is widely cited as the basis for later research on lower ratios and class sizes.14 The study examined 64 day care centers in three cities, and in eight of the centers, students were randomly assigned to classrooms of different sizes and ratios. The authors found that overall, smaller class sizes had significant positive effects on cognitive outcomes (as measured by instruments including the Preschool Inventory, or PSI, and the Peabody Picture Vocabulary Test, or PPVT) for 3- and 4-year-olds, through increased engagement in “creative, verbal/intellectual and cooperative activity” (p. 25). For example, doubling the group size led to a reduction in fall-to-spring growth on the PPVT.iv A smaller sub-study of the NDCS that included infants and toddlers found that lower ratios were associated with higher-quality care in settings with children younger than 3. However, unmeasured center characteristics may have influenced the results because the centers were not randomly selected.v No US studies since then have employed true experimental designs to examine the effects of ratios or group size.
The evidence for how lower ratios and smaller class sizes may influence children’s cognitive and developmental outcomes tends to find null effects or focuses on children ages 3 and older. A 2017 meta-analysis using data from 38 prior studies that focused on centers serving children ages 3 to 5 found that ratios under 7.5:1 and class sizes of 15 or fewer children were associated with small improvements in children’s cognitive outcomes. Changes in ratios and class sizes above those low thresholds, however, were not associated with more positive outcomes.30 The authors found mostly null effects for social-emotional outcomes, but they noted that there were fewer effect sizes available for social-emotional outcomes, limiting statistical power. The results suggest that reducing ratios and class sizes may be most effective when they are already at lower levels. This study was excluded from our evidence review because it did not examine children under age 3.
A 1999 study using a large national, longitudinal dataset found null effects of ratios and group sizes on cognitive and behavioral outcomes for infants and toddlers when controlling for a variety of other factors.A Infants and toddlers were assessed using the Behavior Problems Index (BPI), the Peabody Individual Achievement Tests (PIAT) in mathematics and reading, and the PPVT. Results for the PIAT-Reading were positive and significant in some specifications, but outcomes were null for all assessments when the full range of controls (including mother fixed effects) were included.
A 2000 study following 89 Black children over the first 3 years of life found that those infants and toddlers in classrooms that met recommended ratios for their age experienced higher expressive and receptive vocabulary growth in classes that met the guidelines compared to those that did not.28 Although this is a promising result, the observational nature of the study and small sample size precluded it from inclusion in our evidence review.
Child-to-caregiver ratios may be more strongly linked to the intermediate outcome of positive caregiving than to child developmental outcomes, but insufficient causal evidence exists to confirm this link. A widely cited study by the National Institute of Child Health and Human Development (NICHD) Early Child Care Research Network in 2000 analyzed observational data for over 600 infants and toddlers in a variety of care settings and concluded that “the strongest and most consistent predictor of observed positive caregiving was the child-adult ratio” (p. 131).15 The study did not, however, meet the standards of evidence for this review because of the cross-sectional nature of the analysis. More rigorous research linking ratios and group sizes in centers to caregiving quality and developmental outcomes over time for children under age 3 is needed.
Lower child-to-caregiver ratios and group sizes may produce better health and safety outcomes for infants and toddlers in centers, but no causal research supports this association. Findings from two correlational studies, however, found that settings with fewer children per caregiver and smaller group sizes saw fewer illnesses and safety violations.16,17
Home-based providers are often more difficult to include in research than centers, and the research on ratios and group sizes in family child care homes suffers from many of the same methodological weaknesses that affect studies of centers. Results from two studies that did not meet the standards of evidence for this review found that positive caregiving improved with lower ratios and group sizes in home settings.15,18 Evidence for other outcomes, including stronger attachment between children and caregivers, has also been found in some studies that did not meet causal standards.19
More research is needed on the link between ratios and child outcomes in home-based settings. Most studies examine how ratios or group sizes contribute to process quality in homes, including factors such as positive caregiving, but do not make the ultimate connection to cognitive or social-emotional outcomes.21 A 2002 study of children in home-based settings found that although group sizes that complied with recommended guidelines were associated with more positive caregiving, group size did not significantly predict child outcomes as measured by test scores.18 The small sample size and observational design precluded this study from informing our evidence review.
Very little evidence links child care ratios or group sizes to safety outcomes in home-based settings, but one longitudinal study that analyzed data for over 1,200 children in various care settings found that larger group sizes (up to a threshold of 8 to 10 children) were associated with a greater risk of respiratory infections (19 to 25 percent higher odds), among 1- and 2-year-olds attending home-based and relative care.B This finding suggests that limiting group sizes to recommended caps may reduce the likelihood of illness. No identified studies examined risk of illness by specific child-to-caregiver ratios in home settings. More research on how child-to-caregiver ratios affect health and safety in home-based settings is warranted.
- An impact is considered statistically significant if p<0.05.
- “Quality” in child care is often conceptualized into components of “structural” and “process” quality. Structural features of quality are the aspects of the child care environment that can be legislated or mandated, such as child-to-staff ratios or caregiver education requirements. Process quality refers to the richness of interactions between children and caregivers, or children and their peers, and of the learning experiences and instruction.31
- As discussed and interpreted in Blau (1999).
- See the discussion in Blau (1999).