A meta-analysis of the distributed practice effect was conducted to examine the impact of temporal variables often overlooked in previous reviews. The study analyzed 839 assessments of distributed practice across 317 experiments in 184 articles. It found that inter-study interval (ISI) and retention interval jointly affect final test retention, with maximal retention increasing as retention interval increases. The review highlights areas needing further research and theoretical implications. The distributed practice effect refers to the impact of ISI on learning, measured on subsequent tests. ISI is the interval between study episodes of the same material. Spaced learning involves measurable time lags between study episodes, while massed learning involves no interruptions. The spacing effect refers to enhanced learning during spaced versus massed study episodes, while the lag effect refers to comparisons of different spacing levels. Past quantitative reviews, such as those by Moss (1996), Lee and Genovese (1988), and Donovan and Radosevich (1999), concluded that distributed practice improves retention, though the effect is moderated by variables like task domain and retention interval. Janiszewski et al. (2003) found that longer ISIs increased effect size, while retention interval had no effect. However, Donovan and Radosevich's review suggested that longer ISIs could impair learning, contradicting Janiszewski et al.'s findings. The current meta-analysis examined the joint effects of ISI and retention interval, finding that optimal ISI increases with retention interval. It also found that longer ISIs can be detrimental for skill tasks but beneficial for verbal memory tasks. The study emphasized the importance of considering temporal and procedural variables in distributed practice research. The analysis revealed that studies with varying ISI and retention intervals showed non-monotonic effects of ISI on final-test performance. For retention intervals less than one day, spaced presentations improved performance, while for longer intervals, the effect was less pronounced. The study also highlighted the importance of controlling for relearning confounds in distributed practice research. The results suggest that distributed practice benefits are robust but depend on task type and retention interval. The study concludes that optimal ISI increases with retention interval, and that the joint effects of ISI and retention interval are complex and task-dependent. The findings have implications for both practical applications and theoretical understanding of distributed practice.A meta-analysis of the distributed practice effect was conducted to examine the impact of temporal variables often overlooked in previous reviews. The study analyzed 839 assessments of distributed practice across 317 experiments in 184 articles. It found that inter-study interval (ISI) and retention interval jointly affect final test retention, with maximal retention increasing as retention interval increases. The review highlights areas needing further research and theoretical implications. The distributed practice effect refers to the impact of ISI on learning, measured on subsequent tests. ISI is the interval between study episodes of the same material. Spaced learning involves measurable time lags between study episodes, while massed learning involves no interruptions. The spacing effect refers to enhanced learning during spaced versus massed study episodes, while the lag effect refers to comparisons of different spacing levels. Past quantitative reviews, such as those by Moss (1996), Lee and Genovese (1988), and Donovan and Radosevich (1999), concluded that distributed practice improves retention, though the effect is moderated by variables like task domain and retention interval. Janiszewski et al. (2003) found that longer ISIs increased effect size, while retention interval had no effect. However, Donovan and Radosevich's review suggested that longer ISIs could impair learning, contradicting Janiszewski et al.'s findings. The current meta-analysis examined the joint effects of ISI and retention interval, finding that optimal ISI increases with retention interval. It also found that longer ISIs can be detrimental for skill tasks but beneficial for verbal memory tasks. The study emphasized the importance of considering temporal and procedural variables in distributed practice research. The analysis revealed that studies with varying ISI and retention intervals showed non-monotonic effects of ISI on final-test performance. For retention intervals less than one day, spaced presentations improved performance, while for longer intervals, the effect was less pronounced. The study also highlighted the importance of controlling for relearning confounds in distributed practice research. The results suggest that distributed practice benefits are robust but depend on task type and retention interval. The study concludes that optimal ISI increases with retention interval, and that the joint effects of ISI and retention interval are complex and task-dependent. The findings have implications for both practical applications and theoretical understanding of distributed practice.