10.7910/DVN/BBS0SH Gray-Lobe, Guthrie Guthrie Gray-Lobe https://orcid.org/0000-0002-5276-5860 University of Chicago Harvard Dataverse 2026 Social Sciences Gray-Lobe, Guthrie Guthrie Gray-Lobe University of Chicago 2026-03-23 2026-03-31 /papers/w31853 1052760 application/zip 1.0 Creative Commons CC0 1.0 Universal Public Domain Dedication. This paper tests whether increasing students’ exposure to “desirable difficulties” improves learning in real classrooms. In a year-long field experiment in Nigerian primary schools, interleaved math practice raised short-term test performance by 0.28 standard deviations but had no effect on cumulative end-of-year assessments. Gains were concentrated among lower-achieving students, while higher-achieving students saw little or no benefit. The results suggest that strategies that make learning more effortful can boost short-term mastery but may not produce durable improvements, highlighting the limits of applying laboratory-based cognitive interventions at scale. The data are anonymized.