Randomized Controlled Trial (RCT) of...
Public Montessori Preschools
Reviewed
PNAS (October 2025) published findings from a national RCT of public Montessori preschools. The study abstract claims positive impacts on early reading and cognitive skills at the end of kindergarten but doesn’t mention a key study limitation: Sample loss (“attrition”) of over 80% that differed between the treatment and control groups, rendering these findings only suggestive – not reliable.
Although seminal studies from the early 1960s suggested quality preschool can have lasting positive effects, agreement is lacking on the efficacy of different preschool models. The Montessori model is longstanding but lacks rigorous impact studies; prior random lottery studies included just one or two schools, among other compromises. Here, we report on end-of-kindergarten (age 5 to 6) impacts from a national study of public Montessori preschool. We compared children offered a Montessori seat via competitive lottery admission processes at one of 24 public Montessori schools at age 3 (n = 242) to children not offered a seat (n = 346), estimating Montessori impacts with intention-to-treat and complier average causal effect models. Roughly half of the treatment sample still attended Montessori for kindergarten. Although there were no notable impacts at the end of PK3 or PK4, at the end of kindergarten, controlling for baseline scores and demographics, Montessori children had significantly higher reading, short-term memory, theory of mind, and executive function scores. Intention-to-treat effect sizes exceeded a fifth of a SD, considered large in field-based school research [M. A. Kraft, Educ. Res. 49, 241–253 (2020)]. This contrasts sharply with the more typical finding, where impacts of preschool are observed immediately following the program but disappear by the end of kindergarten. Further, a cost analysis suggested three years of public Montessori preschool costs less per child than traditional programs, largely due to Montessori having higher child:teacher ratios in PK3 and PK4. Although sensitivity and robustness analyses yielded similar results, important limitations of the study should be noted.
Although seminal studies from the early 1960s suggested quality preschool can have lasting positive effects, agreement is lacking on the efficacy of different preschool models. The Montessori model is longstanding but lacks rigorous impact studies; prior random lottery studies included just one or two schools, among other compromises. Here, we report on end-of-kindergarten (age 5 to 6) impacts from a national study of public Montessori preschool. We compared children offered a Montessori seat via competitive lottery admission processes at one of 24 public Montessori schools at age 3 (n = 242) to children not offered a seat (n = 346), estimating Montessori impacts with intention-to-treat and complier average causal effect models. Roughly half of the treatment sample still attended Montessori for kindergarten. Although there were no notable impacts at the end of PK3 or PK4, at the end of kindergarten, controlling for baseline scores and demographics, Montessori children had significantly higher reading, short-term memory, theory of mind, and executive function scores. Intention-to-treat effect sizes exceeded a fifth of a SD, considered large in field-based school research [M. A. Kraft, Educ. Res. 49, 241–253 (2020)]. This contrasts sharply with the more typical finding, where impacts of preschool are observed immediately following the program but disappear by the end of kindergarten. However, these findings are only suggestive because the study had high rates of sample attrition that differed between the treatment and control group, creating substantial risk of bias. Further, a cost analysis suggested three years of public Montessori preschool costs less per child than traditional programs, largely due to Montessori having higher child:teacher ratios in PK3 and PK4. Although sensitivity and robustness analyses yielded similar results, important limitations of the study should be noted.
No-Spin’s Study Overview
An RCT of public Montessori preschools reports positive impacts on early reading and cognitive skills at the end of kindergarten. However, these findings are only suggestive – not reliable – due to sample loss (“attrition”) of over 80% that differed between treatment and control groups.
Program:
- Montessori is a commonly used “alternative” educational model. Montessori classrooms are typically larger and mixed age (e.g., grouping children ages 3 to 6 together) so that older children can serve as role models for younger ones. While teachers provide small group lessons, children learn largely through self-directed use of curated hands-on materials.
Study Design:
- A total of 2,919 children applied to 24 oversubscribed public Montessori schools. The schools used randomized lotteries to determine which children were offered admission (treatment group) or not (control group).
- Of the 588 children whose families consented to study participation, 51% were male, 47% were non-white, and their average age was 3.4 at the time of preschool entry.
Findings:
- Of the nine prespecified primary outcomes at kindergarten follow-up, the study found sizable, statistically significant positive effects on four: one measure of early literacy skills and three measures of cognitive skills (standardized effect sizes ranged from 0.22 to 0.30).
- However, we believe these findings are unreliable due to a key study limitation: High sample attrition that differed between the treatment and control groups. Specifically -
- Only about 34% of treatment group families and 17% of control group families consented to study participation (e.g., to have their child's outcome data collected). The differing consent rates were likely driven by the fact that most families knew whether or not their child had been admitted to Montessori (i.e., was in the treatment group) when they were asked to consent.
- Additional sample loss at the kindergarten follow-up left roughly 18% of the treatment group and 8% of the control group in the final sample with available outcome data to estimate the program’s impacts.
- Such high attrition creates “unacceptable levels of potential bias” under recognized standards (WWC) by undermining the treatment-control group equivalence created by randomization.
Comment:
- The Montessori study is ongoing, with plans to measure impacts on standardized test scores in the 3rd grade. If the researchers are able to measure these outcomes for a high proportion of the randomized sample (e.g., by partnering with school districts to obtain state or district test scores), the study could still produce a reliable estimate of the program’s third-grade impacts.
- Disclosure: Arnold Ventures, which funds No-Spin Evidence Review, helped fund the kindergarten follow-up of the Montessori study.
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