A national randomized controlled trial of the impact of public Montessori preschool at the end of kindergarten
Lillard, A. S., Loeb, D., Berg, J., Escueta, M., Manship, K., Hauser, A., & Daggett, E. D. (2025). Proceedings of the National Academy of Sciences, 122 (43), e2506130122 .
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examining588Students, gradesPK-K
Single Study Review
Review Details
Reviewed: January 2026
- Single Study Review (findings for Montessori preschool—Lillard et al. (2025).)
- Randomized Controlled Trial
- Meets WWC standards with reservations because it is a randomized controlled trial with high attrition, but the analytic intervention and comparison groups satisfy the baseline equivalence requirement.
This review may not reflect the full body of research evidence for this intervention.
Evidence Tier rating based solely on this study. This intervention may achieve a higher tier when combined with the full body of evidence.
Findings
|
Outcome measure |
Comparison | Period | Sample |
Intervention mean |
Comparison mean |
Significant? |
Improvement index |
||
|---|---|---|---|---|---|---|---|---|---|
|
Wechsler intelligence scale for children (WISC-III), Forward Digit Span |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
7.99 |
7.41 |
Yes |
|
||
|
Theory of Mind (ToM) Scale |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
3.20 |
2.95 |
Yes |
|
||
|
Heads-Toes-Knees-Shoulders (HTKS) |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
42.45 |
45.57 |
No |
-- | ||
|
Wechsler intelligence scale for children (WISC-III), Backward Digit Span |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
3.24 |
2.92 |
No |
-- | ||
| Show Supplemental Findings | |||||||||
|
Wechsler intelligence scale for children (WISC-III), Backward Digit Span |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-1 Years |
Full sample (end of prekindergarten-4);
|
1.89 |
1.65 |
No |
-- | ||
|
Wechsler intelligence scale for children (WISC-III), Forward Digit Span |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-1 Years |
Full sample (end of prekindergarten-4);
|
6.79 |
6.66 |
No |
-- | ||
|
Wechsler intelligence scale for children (WISC-III), Forward Digit Span |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
5.40 |
5.30 |
No |
-- | ||
|
Wechsler intelligence scale for children (WISC-III), Backward Digit Span |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
0.68 |
0.64 |
No |
-- | ||
|
Theory of Mind (ToM) Scale |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
1.40 |
1.39 |
No |
-- | ||
|
Theory of Mind (ToM) Scale |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-1 Years |
Full sample (end of prekindergarten-4);
|
2.17 |
2.17 |
No |
-- | ||
|
Outcome measure |
Comparison | Period | Sample |
Intervention mean |
Comparison mean |
Significant? |
Improvement index |
||
|---|---|---|---|---|---|---|---|---|---|
|
Woodcock-Johnson Test of Achievement-III - Applied Problems |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
0.82 |
0.67 |
No |
-- | ||
| Show Supplemental Findings | |||||||||
|
Woodcock-Johnson Test of Achievement-III - Applied Problems |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
0.84 |
0.81 |
No |
-- | ||
|
Woodcock-Johnson Test of Achievement-III - Applied Problems |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-1 Years |
Full sample (end of prekindergarten-4);
|
0.82 |
0.85 |
No |
-- | ||
|
Outcome measure |
Comparison | Period | Sample |
Intervention mean |
Comparison mean |
Significant? |
Improvement index |
||
|---|---|---|---|---|---|---|---|---|---|
|
Woodcock-Johnson Test of Achievement-III - Letter-Word Identification |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
1.37 |
1.00 |
Yes |
|
||
| Show Supplemental Findings | |||||||||
|
Woodcock-Johnson Test of Achievement-III - Letter-Word Identification |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-1 Years |
Full sample (end of prekindergarten-4);
|
0.89 |
0.57 |
No |
-- | ||
|
Woodcock-Johnson Test of Achievement-III - Letter-Word Identification |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
0.62 |
0.62 |
No |
-- | ||
|
Outcome measure |
Comparison | Period | Sample |
Intervention mean |
Comparison mean |
Significant? |
Improvement index |
||
|---|---|---|---|---|---|---|---|---|---|
|
The Social Problem-Solving (SPS) Test–Revised |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
1.26 |
1.47 |
No |
-- | ||
| Show Supplemental Findings | |||||||||
|
The Social Problem-Solving (SPS) Test–Revised |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
0.77 |
0.69 |
No |
-- | ||
|
Outcome measure |
Comparison | Period | Sample |
Intervention mean |
Comparison mean |
Significant? |
Improvement index |
||
|---|---|---|---|---|---|---|---|---|---|
|
Woodcock-Johnson Test of Achievement-III - Picture Vocabulary |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
0 Days |
Full sample (end of Kindergarten);
|
0.26 |
0.19 |
No |
-- | ||
| Show Supplemental Findings | |||||||||
|
Woodcock-Johnson Test of Achievement-III - Picture Vocabulary |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-2 Years |
Full sample (end of prekindergarten-3);
|
0.54 |
0.45 |
No |
-- | ||
|
Woodcock-Johnson Test of Achievement-III - Picture Vocabulary |
Montessori preschool—Lillard et al. (2025). vs. Business as usual |
-1 Years |
Full sample (end of prekindergarten-4);
|
0.32 |
0.37 |
No |
-- | ||
Evidence Tier rating based solely on this study. This intervention may achieve a higher tier when combined with the full body of evidence.
Sample Characteristics
Characteristics of study sample as reported by study author.
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16% English language learners -
Other or unknown: 100% -
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Midwest, Northeast, West
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Race Asian 6% Black 15% Other or unknown 37% White 42% -
Ethnicity Hispanic 27% Other or unknown 73% -
Eligible for Free and Reduced Price Lunch Free or reduced price lunch (FRPL) 28% Other or unknown 72%
Study Details
Setting
The study took place in 24 oversubscribed public Montessori schools across the United States, located across New England, the Mid-Atlantic, the Midwest, and the western U.S. Each school operated Montessori classrooms serving ages 3–6.
Study sample
The researchers compared outcomes for children offered Montessori seats starting in preschool-age-3 (PK3) via a competitive lottery admission (707 children) to a comparison group of students that were not offered a seat in the lotteries (2,097 children). The researchers were able to gain consent from and include 588 of the children in the study: 242 offered Montessori placement and 346 that were not. Study children were followed from PK3 through kindergarten. Most children in the study reported their race and ethnicity as either White (42%) or Black (15%). Twenty-seven percent were Hispanic. Most (84%) spoke English at home. Sixty-one percent of families in the study had a family income under $75,000.
Intervention Group
Children in the intervention condition were offered admission via lottery to a public Montessori seat in PK3 and, if they enrolled, attended a mixed-age Montessori Primary classroom serving ages 3–6. Study schools met minimum Montessori implementation criteria including Montessori-trained lead teachers, a trained teacher plus a non-teaching assistant (no more than two adults), Montessori lesson materials, and at least a 2-hour uninterrupted daily free-choice work period. The intervention could last up to three school years (PK3, PK4, and kindergarten). Eighty-eight percent of those offered a Montessori seat in PK3 accepted. Almost half (49%) attended a Montessori school through kindergarten.
Comparison Group
The comparison condition was business-as-usual schooling for children who did not win a Montessori seat. Most attended non-Montessori public programs (traditional district, charter, or magnet), some attended private programs, a small share were not in school, and some enrollment was unknown. Nineteen percent of comparison group children attended a Montessori school through a non-lottery enrollment and 11% attended a Montessori school through kindergarten.
Support for implementation
Schools were required to meet minimum implementation criteria tied to the Association Montessori Internationale (AMI) and American Montessori Society (AMS). This included trained lead teachers, a trained lead teacher plus a non-teaching assistant (no more than two adults), mixed-age (3–6) classrooms, a daily 2-hour uninterrupted free-choice work period, and access to a largely complete set of Montessori Primary materials (with minimal non-Montessori materials). Montessori implementation was supported through teacher Montessori certification/training (AMI/AMS) and ongoing professional development requirements (reported as similar for Montessori and traditional pre-k teachers in district administrator interviews).
An indicator of the effect of the intervention, the improvement index can be interpreted as the expected change in percentile rank for an average comparison group student if that student had received the intervention.
For more, please see the WWC Glossary entry for improvement index.
An outcome is the knowledge, skills, and attitudes that are attained as a result of an activity. An outcome measures is an instrument, device, or method that provides data on the outcome.
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The sample on which the analysis was conducted.
The group to which the intervention group is compared, which may include a different intervention, business as usual, or no services.
The timing of the post-intervention outcome measure.
The number of students included in the analysis.
The mean score of students in the intervention group.
The mean score of students in the comparison group.
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Study findings for this report.
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as defined in the
non-regulatory guidance for ESSA
and the regulations for ED discretionary grants (EDGAR Part 77).