Spaced repetition
Each successful review buys you a longer next interval.
definition
Scheduling reviews at expanding intervals to counter the forgetting curve.
Spaced repetition is the practice of reviewing material at expanding intervals — 1 day, then 3, then 7, then 21, then 60 — instead of massing all review together in one session. It's the operational answer to Ebbinghaus's 1885 discovery that memory decays exponentially without reactivation.
The counter-intuitive property is that longer gaps between reviews produce BETTER long-term retention than shorter gaps. A student who reviews material once a day for 10 days will retain less at 30-day follow-up than a student who reviews the same material on day 1, day 3, day 10, and day 25. Same number of reviews. Different schedule. Dramatic retention difference.
SM-2 is the specific scheduling algorithm published by Piotr Wozniak in 1990 that most modern spaced repetition software (Anki, RemNote, Brainback) descends from. It adjusts intervals based on your grade after each review — hard cards come back sooner, easy cards come back later, and everything self-tunes to your specific forgetting curve.
Brainback runs SM-2 in production with one key difference from Anki: the cards are generated automatically from your own mistakes and low-scored explain-backs. You don't build a deck; the deck builds itself as you use the product. Every card tests a gap you actually have, not a generic textbook fact.
What the research says about spaced repetition.
Three papers that established the finding.
Every design decision Brainback makes about spaced repetition flows from these papers. Cited so you can verify.
Memory: A Contribution to Experimental Psychology
First mapped the forgetting curve. Nonsense-syllable retention drops to ~40% within 24 hours without review, then decays more slowly toward ~15% at 1 week. The curve has been reproduced thousands of times across populations and material types.
Effect of Delay of Reinforcement on Learning and Retention
Established that spaced practice produces stronger conditioning than massed practice. First formal demonstration of what we now call the spacing effect.
Optimization of Learning: SuperMemo Algorithm SM-2
Formalized the algorithm that expands intervals based on user-graded difficulty. SM-2 remains the base algorithm for most consumer spaced-repetition software 35 years later.
How it actually works, at the cognitive and neural level.
The mechanism isn’t decorative. Understanding WHY the effect exists lets you tell when it applies and when it doesn’t.
The neurobiological mechanism is called 'consolidation.' Freshly formed memories start in the hippocampus and get gradually transferred to the neocortex over days to weeks. Each successful retrieval during the consolidation window strengthens the neocortical representation. If the retrieval happens too early (before decay), the plasticity is minimal; too late (after decay), the memory has to be re-encoded from scratch.
The 'right moment' to review is at the tail end of the forgetting curve — when the memory is still retrievable but starting to fade. Reviewing at that moment produces the strongest reconsolidation. SM-2 approximates this timing by tracking your grade history and extending the interval after each successful review.
The compounding effect matters: each successful review at the extended interval buys you an even longer next interval. Over months, well-learned cards drift out to 90-180-day intervals with minimal effort. Over a full year, ~200 cards can be maintained at 90%+ retention with 5-10 minutes of daily review.
Same scenario. Two study strategies.
Med school · pharmacology.
You need to remember the mechanism of action, side effects, and contraindications for ~200 drugs by Step 1 next year.
You make flashcards for each drug. You review them all daily for a week. By week 2, you're tired and stop. By month 2, you've forgotten most of them. By Step 1, you're re-learning from scratch under exam pressure.
You add each drug as a Brainback card as it comes up in class. Day 1 review, day 3, day 7, day 21, day 60, day 180. By month 6, you're reviewing ~20 cards a day (5 minutes). By Step 1, all 200 drugs are at 90%+ retention with 10 minutes of daily maintenance.
The compounding effect: 5-10 minutes a day, sustained for 12 months, produces reliable long-term retention of 200+ items. The alternative — cramming — produces short-term retention that evaporates within weeks. Same total effort. Dramatically different outcomes.
The method, made operable.
SM-2, the 1990 algorithm every major SRS descends from. Cards get scheduled based on your grade (Again / Hard / Good / Easy) and an ease factor that adjusts over time. Cards you know well drift to weekly, then monthly, then quarterly reviews; cards you struggle with come back tomorrow.
Three subject areas where the effect is strongest.
The method works everywhere. It works especially well in these three domains — for reasons specific to each.
Volume of discrete facts that need reliable retrieval on demand. Anki + med school is a cliché because it works.
Word-meaning pairs are the archetypal SRS card. Duolingo works partly because it's a diluted SRS.
Case name → holding is a fact pair that needs long-term retention through the bar exam.
Three weaker approaches — and why they underperform.
Understanding what the method beats (and by how much) tells you when to reach for it.
why weaker · Short-term retention only. Everything crammed for tomorrow's exam is gone within 2 weeks. Sunk cost.
why weaker · Grinding a full deck daily is Anki-style rot. The whole point is the algorithm decides what returns when — daily-everything review defeats the spacing effect.
why weaker · Self-assessment of memory strength is unreliable. Students consistently over-estimate what they know. The algorithm doesn't have that bias.
How to actually do it tomorrow morning.
5-15 minutes daily, at whatever time you can maintain daily. The steps below are the sustainable protocol — not a heroic one-off.
Open the recall queue at the same time each day (morning coffee works well).
Review cards in the order the algorithm serves them. Don't re-order.
For each card: attempt retrieval, flip, grade honestly (Again/Hard/Good/Easy).
Grade based on effort, not outcome — 'I got it but had to think' is Hard, not Good.
Stop when the queue is empty. If the queue is empty, don't invent more.
Skip days if you must; the algorithm handles the reschedule. Don't skip more than 3 days in a row.
Reviewing everything every day. Spaced repetition works precisely because you're NOT reviewing what you already know. Grinding a full deck daily is Anki-style rot; the whole point is the algorithm decides what returns when.
The mechanism, in one paragraph.
Each retrieval right before decay resets the forgetting curve — and each successful retrieval widens the next interval before decay resumes. Over months, this compounds: material learned in September is still retrievable in May with a fraction of the review effort of nightly re-reading.
Everything students ask about spaced repetition.
Is this different from Anki?▾
Same algorithm (both use SM-2 lineage). Anki asks you to make your own deck. Brainback generates cards from your mistakes automatically. Different card-source strategies, same scheduling math.
How long until it 'works'?▾
Weeks 1-3: too early to tell. Weeks 4-8: retention shifts noticeably. Weeks 12+: the compounding is undeniable.
What if I miss a day?▾
The queue holds. Cards you were due to review pile up but they don't get 'stale' — SM-2 handles the reschedule. Missing a week is fine; missing a month means some cards drift back to shorter intervals.
Should I use Anki alongside Brainback?▾
Depends. For a pre-built domain deck (First Aid for USMLE, AnKing) — Anki still wins. For cards generated from YOUR specific mistakes — Brainback wins. Many med students use both.
What's the difference between spaced repetition and active recall?▾
Active recall = the retrieval act. Spaced repetition = the timing schedule for retrieval acts. You do active recall AT spaced intervals. Both are load-bearing.
Three more methods that stack with this one.
spaced repetition
Try the method in the product.
Spaced recall runs on every account, free tier included.