A Randomized Controlled Trial (RCT) of...
Full-Day Versus Half-Day Pre-K For Primarily Low-Income Children
Reviewed
AERA Open (February 2026) published findings of a high-quality RCT of full-day (as compared to half-day) pre-K among primarily low-income, Hispanic children. The paper reports that full-day pre-K reduced students’ English language learner (ELL) designations in grades K-3 but doesn’t note an important caveat: ELL was not prespecified as a primary study outcome, so these findings are only suggestive. Meanwhile, the study’s primary outcomes (e.g., 3rd grade math and reading scores) have not yet been reported, preventing reliable conclusions about program effectiveness.
This study estimates the causal effect of randomized offers of full-day versus half-day pre-K on students’ likelihood of having English language learner (ELL) designations in early elementary grades. We leverage a randomized, controlled trial in a Colorado district serving primarily low-income Latinx families, where students assigned to full-day pre-K received more than twice as much instructional time. Although instruction was not formally multilingual, we hypothesize that additional English exposure in full-day classrooms may reduce the likelihood of a later ELL designation. Among students likely not fluent in English at pre-K entry, full-day pre-K offers reduce ELL designations in grades K–3 by 8–16 percentage points. These findings contribute to evidence on the long-term benefits of full-day pre-K and suggest the added costs of full- versus half-day pre-K may be offset by reduced need for ELL services in the early grades—an important consideration for district leaders weighing the value of expanding program duration.
In this secondary analysis of a randomized controlled trial, we
This study estimates the causal effect of randomized offers of full-day versus half-day pre-K on students’ likelihood of having English language learner (ELL) designations in early elementary grades. We leverage a randomized, controlled trial in a Colorado district serving primarily low-income Latinx families, where students assigned to full-day pre-K received more than twice as much instructional time. Although instruction was not formally multilingual, we hypothesize that additional English exposure in full-day classrooms may reduce the likelihood of a later ELL designation. Among students likely not fluent in English at pre-K entry, full-day pre-K offers reduce ELL designations in grades K–3 by 8–16 percentage points. These findings contribute to evidence on the long-term benefits of full-day pre-K
but, as secondary results, should be considered tentative pending replication. If confirmed, they
and suggest the added costs of full- versus half-day pre-K may be offset by reduced need for ELL services in the early grades – an important consideration for district leaders weighing the value of expanding program duration.
No-Spin’s Study Overview
High-quality RCT of full-day (as compared to half-day) pre-K among primarily low-income, Hispanic children has not yet reported findings on its prespecified primary outcomes in 3rd grade, preventing reliable conclusions about program effectiveness. Analysis of secondary outcomes finds suggestive evidence of reductions in students’ English language learner (ELL) designations in grades K-3 that may warrant examination in future research.
Program and Study Design:
- Over three school years, Westminster, Colorado public schools used a randomized lottery to assign children to half-day pre-K (three hours per day, four days per week for a total of 12 hours weekly) versus full-day pre-K (six hours per day, five days per week for a total of 30 hours).
- A lottery was used because demand for full-day pre-K exceeded the number of full-day slots available. 90% of families that won the lottery enrolled their child in full-day pre-K.
- The study sample comprised 795 children. They averaged 4.5 years of age at baseline, 86% were from low-income families, and 70% were Hispanic.
- Based on our review criteria, this was a high-quality RCT.
Findings:
- The study has not yet reported findings on its prespecified primary outcomes – 3rd grade math and English language arts test scores and K-3 attendance – preventing reliable conclusions about program effectiveness.
- The AERA Open paper reports on an outcome prespecified as secondary – ELL designations in K-3 – and finds that full-day pre-K reduced such designations (27% of full-day students were ELL-designated vs. 35% of half-day students in 3rd grade, a statistically significant difference).
- A separate JREE paper reports on another secondary outcome – special education placements in K-3 – and finds reductions among a subgroup of at-risk students in grades K-2.
- However, these secondary findings are only suggestive – not yet reliable – under established scientific standards (IES, FDA), as they could be chance findings stemming from researchers’ examination of multiple outcomes and subgroups.
Comment:
- Arnold Ventures, which funds No-Spin Evidence Review, helped fund this RCT of half-day vs. full-day Pre-K.
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