01
Forgetting is fastest immediately after learning, then slows down
Plain-language finding: A 2015 replication reproduced the steep early drop and later flattening of Ebbinghaus' forgetting curve.
Ebbinghaus established this in 1885 by memorising nonsense syllables and measuring how much relearning effort was saved at delays from twenty minutes to a month. The curve drops steeply at first and then flattens. A 2015 replication reproduced the original result more than a century later.
Study: Murre, J. M. J., and Dros, J. (2015). Replication and Analysis of Ebbinghaus' Forgetting Curve. PLOS ONE, 10(7).
What it means in practice: Most of what you lose from a lesson, you lose in the first day or two. That is an argument for revisiting material soon after the first exposure, while it is still cheap to do.
02
Spreading study across sessions beats massing it, across 317 experiments
Plain-language finding: A 2006 meta-analysis covering 839 assessments from 317 experiments found a reliable advantage for spaced over massed study.
A meta-analysis examined 839 assessments of distributed practice drawn from 317 experiments across 184 articles. It found a reliable benefit for spaced over massed study, and that the gap producing the best retention grew as the delay before the final test grew.
Study: Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., and Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132, 354 to 380.
What it means in practice: Spreading study across sessions often improves later recall compared with concentrating it in one session. The best spacing depends on the task and how long you need to remember it.
03
The best gap between reviews is a proportion of how long you need to remember
Plain-language finding: In a study of more than 1,350 participants, the best review gap changed with how long learners needed to remember the material.
More than 1,350 people learned a set of facts, reviewed them after a gap of up to 3.5 months, and were tested up to a year later. Increasing the gap first improved and then reduced final performance, and the optimal gap declined from roughly 20 to 40% of a one-week delay to roughly 5 to 10% of a one-year delay.
Study: Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., and Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19, 1095 to 1102.
What it means in practice: If you want to still know something in a year, revisit it around a month later. If you need it for something next week, review after a day or two. There is no single correct interval, only one relative to your deadline.
04
Being tested beats rereading, but only when you measure later
Plain-language finding: In a 2006 study, restudying performed better after five minutes, while retrieval practice performed better after two days and one week.
Students read prose passages then either restudied them or took recall tests without feedback. On a final test after five minutes, restudying won. On final tests after two days and one week, the students who had been tested retained substantially more. The advantage of testing is invisible if you measure immediately.
Study: Roediger, H. L., and Karpicke, J. D. (2006). Test-Enhanced Learning: Taking Memory Tests Improves Long-Term Retention. Psychological Science, 17(3).
What it means in practice: A quiz at the end of a lesson is not assessment, it is the part that makes the lesson stick. It also explains why rereading feels more effective than it is: it wins the short-term comparison you can feel.
