This paper presents a meta-analysis of distributed practice in verbal recall tasks, examining the effects of inter-study intervals (ISIs) and retention intervals on learning outcomes. The authors reviewed 839 assessments from 317 experiments across 184 articles, focusing on the spacing effect (consecutive massed presentations vs. spaced learning episodes) and lag effect (less spaced vs. more spaced learning episodes). Key findings include: 1. **Spacing Effect**: Spaced learning episodes significantly improved final-test performance compared to massed presentations, with a 9% improvement for retention intervals less than one minute. 2. **Lag Effect**: The optimal ISI for maximal retention increased as the retention interval increased. For retention intervals of one day, the optimal ISI was around 15 minutes to 1 day. For longer retention intervals, the optimal ISI was longer. 3. **Experimental Design Issues**: The study identified confounding factors in some studies, such as the number of relearning trials and the lack of feedback, which could affect the results. Studies with non-confounded designs generally showed similar patterns of optimal ISI and retention interval. 4. **Expanding vs. Fixed ISIs**: The optimal ISI for maximizing memory was longer for longer retention intervals, supporting the idea that expanding ISIs can enhance learning. The authors conclude that the joint effects of ISI and retention interval are complex and non-monotonic, with optimal ISIs increasing as retention intervals increase. This has implications for both practical applications and theoretical understanding of distributed practice.This paper presents a meta-analysis of distributed practice in verbal recall tasks, examining the effects of inter-study intervals (ISIs) and retention intervals on learning outcomes. The authors reviewed 839 assessments from 317 experiments across 184 articles, focusing on the spacing effect (consecutive massed presentations vs. spaced learning episodes) and lag effect (less spaced vs. more spaced learning episodes). Key findings include: 1. **Spacing Effect**: Spaced learning episodes significantly improved final-test performance compared to massed presentations, with a 9% improvement for retention intervals less than one minute. 2. **Lag Effect**: The optimal ISI for maximal retention increased as the retention interval increased. For retention intervals of one day, the optimal ISI was around 15 minutes to 1 day. For longer retention intervals, the optimal ISI was longer. 3. **Experimental Design Issues**: The study identified confounding factors in some studies, such as the number of relearning trials and the lack of feedback, which could affect the results. Studies with non-confounded designs generally showed similar patterns of optimal ISI and retention interval. 4. **Expanding vs. Fixed ISIs**: The optimal ISI for maximizing memory was longer for longer retention intervals, supporting the idea that expanding ISIs can enhance learning. The authors conclude that the joint effects of ISI and retention interval are complex and non-monotonic, with optimal ISIs increasing as retention intervals increase. This has implications for both practical applications and theoretical understanding of distributed practice.