Theoretical Analysis and Practical Implications

More than a century of research shows that increasing the gap between study episodes using the same material can enhance retention, 10 yet little is known about how this so-called distributed practice effect unfolds over nontrivial periods. In two three-session laboratory studies, we 11 examined the effects of gap on retention of foreign vocabulary, facts, and names of visual objects, with test delays up to 6 months. An optimal 12 gap improved final recall by up to 150%. Both studies demonstrated nonmonotonic gap effects: Increases in gap caused test accuracy to initially 13 sharply increase and then gradually decline. These results provide new constraints on theories of spacing and confirm the importance of 14 cumulative reviews to promote retention over meaningful time periods.