Balancing Spacing and Repetition for Time-Constrained Learning

Spaced retrieval enhances retention but is often time-intensive, limiting its practical use in educational settings. When study time is constrained, should learners prioritize fewer, widely spaced repetitions or more repetitions with shorter intervals? We tested spacing, repetition, and boundary-condition accounts by manipulating item difficulty and practice schedules across three experiments (N = 1,944). Participants learned Indonesian vocabulary under four conditions that traded off repetition frequency and spacing interval. Results showed that increasing repetitions at the expense of wider spacing improved immediate performance and delayed retention, particularly for difficult items, whereas easy items reached an asymptote after a moderate number of repetitions. These findings support a boundary-condition account, indicating that the effectiveness of spacing and repetition depends on item difficulty and sufficient successful retrievals. Together, repetition, spacing, and item difficulty jointly regulate retrieval success. The results refine retrieval-based learning theory and offer guidance for efficient study under time constraints.