In one sentence
A meta-analysis of applied classroom studies found a moderate advantage for distributed over massed practice (d = 0.54) — not only in decontextualised lab drills.
What the researchers did
Mawson and Kang noted that most classic spacing evidence comes from laboratories with materials unlike school curricula. They ran a systematic review of applied research on distributed practice in classrooms, keeping only studies whose materials and timescales fit curriculum-based learning.
Screening over 3000 articles yielded 22 reports with 31 effect sizes and N > 3000 learners. They meta-analysed the overall distributed-versus-massed contrast. A full quantitative moderator analysis was limited by study count, so patterns by retention interval, education level, and re-exposure count are described more cautiously.
What they found
- Overall effect favoured distributed over massed practice: d = 0.54 (95% CI 0.31–0.77) — a moderate classroom-relevant gap.
- Generally larger effects tended to appear with longer retention intervals.
- Larger effects also tended to appear at higher education levels.
- Studies with fewer re-exposures to the materials also tended toward larger effects (pattern note, not a definitive moderator test).
What this means for learners and educators
- Treat “spread reviews across days/weeks” as a default classroom design, not a lab curiosity — the applied pool still shows a clear average benefit.
- Plan for longer retention checks (unit tests weeks later) where spacing’s edge often shows most clearly.
- Avoid stuffing many near-identical re-exposures into a short window and calling it spaced — fewer, well-timed returns may matter more than endless same-day loops.
- University and later schooling may see especially usable gains, but the review still speaks to curriculum settings broadly.
Limitations and what we don't know yet
Thirty-one effects is enough for a headline average but thin for confident moderator maps — the authors flag that a comprehensive quantitative moderator analysis was not possible. Classroom studies vary in subjects, schedules, and what “massed” means. Publication bias and uneven quality remain risks in any applied meta-analysis. The CI (0.31–0.77) shows uncertainty about exact size. Optimal gaps between sessions for each subject are not settled here.