Spaced Repetition — Body Handbook

Productivity · §704

Spaced Repetition

Most of what you learn is forgotten within a year. Spaced repetition is software that schedules flashcards to come back just before you'd lose them — you spend less time per fact than you would re-learning it later, and you keep the knowledge for decades instead of weeks. The basic effect has more than a century of lab evidence behind it, and the technique is now standard equipment for medical students, language learners, and anyone whose work depends on remembering things. The catch is the daily discipline — fifteen to thirty minutes a day, every day, for as long as you want to remember the material.

Do · Daily Evidence Strong Chapter Productivity

The wins are real and the catch is the daily discipline. Commit twenty minutes a day and you keep most of what you study for years — language words, medical anatomy, legal cases, programming syntax, anything that fits on a card. The cost is daily contact, and the failure mode is the queue piling up the week you skip it. The evidence behind it is one of the deepest in learning science; the only real question is whether you need long-term recall enough to earn back the time.

You know how the language vocabulary you memorised for a high school exam is gone now? That's the math at work. Hermann Ebbinghaus measured it in 1885, and a careful 2015 replication reproduced his curve almost exactly — without rehearsal, about 60–70% of new facts vanish in the first day, and the curve keeps eroding for weeks Murre & Dros 2015. What spaced repetition does is hand you the same card again precisely when you'd be about to lose it. Each successful catch strengthens the memory more than the previous one did, and the next-due date moves out further. Five reviews over six months beats fifty re-reads in one weekend, for reasons documented across 184 articles and 317 separate experiments by 2006 Cepeda et al. 2006.

The active-recall half matters as much as the spacing. Re-reading a textbook page feels productive and barely is. Closing the book and trying to recall the page is what actually files it away.

One input sits underneath all of it: sleep. The review triggers consolidation, but the night is when it actually sets — so chronically short nights, the sleep-debt state, quietly blunt the very filing your cards are paying for.

What the long-term numbers look like

The most striking data comes from Harry Bahrick, who in 1984 tested 733 American adults on Spanish vocabulary — some of them had learned it in high school 50 years earlier. Beyond an initial 3–6 years of decline, words entered what he called permastore: a flat shelf of vocabulary that stayed accessible for 25-plus years without rehearsal. The strongest predictor of what was still there was how deeply it was learned the first time around Bahrick 1984. In a follow-up, 13 review sessions spaced 56 days apart produced the same five-year retention as 26 sessions spaced 14 days apart — half the work, same result Bahrick et al. 1993.

The cleanest real-world test is medical school. At Washington University, researchers controlled for MCAT scores, second-year grades, and test anxiety, then asked what additional study habits predicted USMLE Step 1 performance. Each additional 1,700 Anki cards a student had reviewed was independently worth one extra point on the exam Deng et al. 2015. A 2023 cohort study at the University of Minnesota found Anki users scored 6–13% higher across four medical school exams, including the closest available proxy for Step 1, and the benefit held regardless of underlying test-taking skill Wothe et al. 2023. Outside Anki, a randomised trial of spaced clinical-education emails — same effect, lower friction — showed retained knowledge in the intervention group versus controls Kerfoot et al. 2007.

The effect is not subtle. The question for anyone with a serious volume of facts to memorise is not whether to use spaced repetition but which tool to use.

How to actually do it

The dominant tool is Anki: free on Windows, Mac, Linux, and Android; $25 one-time on iPhone; free sync across devices. The developer doesn't run it as a business, so there's no advertising and no upsell. Pick Anki unless you have a specific reason not to.

Algorithm. Anki 23.10 (November 2023) and later default to FSRS — the new scheduler that needs roughly 20–30% fewer reviews than the older SM-2 algorithm for the same retention. Set desired retention to 0.90. Only push higher when a high-stakes exam is under a month away.
New cards per day. 10–20 for hobby learners; 20–50 for full-time students. Ten cards a day is about 3,600 a year — roughly the high-frequency vocabulary of a language.
Session length. Fifteen to thirty minutes a day. Past an hour, drop-out risk climbs sharply.
One fact per card. No lists. No "the three causes of X" — the cue itself becomes the answer.
Use a community deck for content you don't own. AnKing for medical school, frequency-ordered decks for languages. Save card-making effort for material specific to your own work.
Pair it with a daily anchor. Morning coffee, commute, lunch. Mobile is the surface that makes consistency possible — a queue checked at the bus stop is the difference between a habit that holds and one that drifts.

The earlier you start, the better the compound. In the Minnesota data, students who began Anki in first year outperformed late starters even after controlling for total card count Wothe et al. 2023. The scheduler needs months of history to work well; jumping in two weeks before an exam gives you the testing-effect benefit without the spacing benefit.

What most guides get wrong

"More cards is more learning." Past your daily time ceiling, every extra card you add now is a review you owe yourself in two months. Cohorts with 30,000-plus cards underperforming peers with 10,000 well-curated ones is a recurring pattern in medical-school communities. The dose-response in the Deng paper is a population average, not a personal optimiser Deng et al. 2015.

"Anki teaches you the language." Anki holds onto facts you already learned somewhere else. Holding a conversation, writing a clinical note, debugging production code — these need reading, speaking, and problem-solving on top of the cards. Spaced repetition is the long-term storage layer; it isn't the learning by itself.

"I'll catch up on the weekend." The whole point of the scheduler is that it shows you each card at the moment closest to when you'd forget it. Skipping days pushes cards past that moment; you're now re-learning them more or less from scratch. The math doesn't reward the catch-up session.

Where it goes wrong

Almost every "I tried Anki and quit" story fits one of four patterns.

Review hell. Three days of life happens. You open the app to 540 cards due. The session feels like a punishment, you bail, and you don't come back. The fix is a low daily new-card cap — lower than feels heroic when you set it — so the queue never grows past what you can clear in twenty minutes on a normal day.
Card hoarding. The deck grows to thousands of cards the learner doesn't actually care about. Review time balloons, the important cards drown, and the streak becomes a chore. The fix is honesty about what's worth remembering, and willingness to delete.
Bad cards. A card whose cue is ambiguous ("Spanish: door") or whose answer is a list keeps failing — Anki flags these as leeches. They eat sessions. The fix is to rewrite or delete the moment a card has failed three or four times in a row.
Recognition, not recall. Studying a pre-made deck of facts you never properly encountered, and learning the wording of the card instead of the knowledge underneath. The fix is to do real first-pass learning — read the chapter, watch the lecture, write your own card — before the deck starts.

What else could you use

For casual learning — fifty Italian phrases for a holiday, a personal-interest topic — the overhead of card-making and daily review usually isn't worth the win. Duolingo, Babbel, and similar apps embed lighter spacing inside a friendlier shell; the ceiling is lower but you actually open them. SuperMemo is the original tool, paid, built on Piotr Wozniak's own algorithms — devoted following, steeper learning curve, harder to extend. RemNote and Obsidian-with-Anki integrations let cards live inside your notes rather than in a separate deck, which suits some workflows.

For high-stakes exams that test application more than recall — bar exam, math olympiads, the clinical Step 2 — spaced past-paper practice often beats card-based study. Same underlying machinery (spacing plus active retrieval), different unit of practice Pan & Rickard 2018.

What changes if you stick with it

The first thing you notice is at the two-week mark: cards you saw on day one come back and you actually remember them. It's a quiet shift but a real one. You used to study, forget, restudy, forget. Now you study, see the card again later, and it's there.

At six months, the deck holds a few thousand items you've genuinely retained. The foreign language you'd put down a decade ago is functional again. The textbook chapter you read in February is recallable in August. The clinical edge cases you crammed for last quarter's rotation are still available now. People around you notice the shift before you do — you stop reaching for your phone to look things up mid-conversation; the language sentence that used to stall by the third word now finishes.

At the year scale, Bahrick's permastore data starts to look less like a curiosity and more like a description of your own knowledge: things you reviewed enough have settled into a state where forgetting roughly stops Bahrick 1984 Bahrick et al. 1993. The cost is the daily contact you've kept up. The win is a different relationship with what you've studied — what you put effort into remembering, you actually remember.

The broader study-skills literature — retrieval practice without software, the testing effect as a daily habit, the spacing effect inside curricula — works on the same machinery. Note-taking systems that pair with cards (Obsidian-Anki, RemNote) are worth a look if your work already lives in notes. Language learning beyond cards (comprehensible input, conversation practice) is the other half of the language story; this article is only the storage layer.

Related in the handbook

Helps

— The review cements a memory, but sleep is what files it — chronically short nights blunt the consolidation your cards depend on.

— Both are daily-discipline productivity tools — one protects focus, the other protects memory.
— Reading is how you take material in; spaced repetition is how you keep it from leaking back out.

Substance + claimed effects

Spaced repetition is a learning technique in which review of material is scheduled at expanding intervals timed to occur just before the learner would otherwise forget it. In modern practice it is operationalised through software — Anki, SuperMemo, RemNote, Quizlet — that holds a deck of digital flashcards, prompts the learner to recall each card, and uses their self-rated success or failure to schedule the next showing. Two cognitive-science findings underpin it: the spacing effect (spaced review beats massed review for long-term retention Cepeda et al. 2006) and the testing effect (active retrieval beats passive re-reading Roediger & Karpicke 2006, Karpicke & Roediger 2008).

The substance is daily use of an SRS over months to years for memorisation-heavy material — most commonly foreign-language vocabulary, medical-school facts, professional certifications, programming syntax, and any other domain with a large stock of arbitrary atomised facts. Effects to cover: long-term retention of memorised content (the dominant outcome), efficiency of study time per fact retained, the felt experience of recall confidence in the moment knowledge is needed, and the consistency of daily learning habits the software's queue enforces. Body-axis effects are minimal — this is a cognitive-tool entry, not a health intervention.

Evidence by addressing question

mechanism

The scheduler exploits two well-replicated memory phenomena.

The forgetting curve. Ebbinghaus (1885) showed memory for nonsense syllables decays roughly exponentially over hours and days; Murre and Dros replicated his protocol on a 22-year-old subject across 70 hours of testing and reproduced the curve with R² = 0.988 Murre & Dros 2015. Without rehearsal, roughly 60–70% of new arbitrary material is forgotten within 24 hours; the curve flattens but does not zero out. The modern replication added a small "bump" at the 24-hour mark consistent with sleep consolidation, but otherwise the 140-year-old shape held.

The spacing effect. Cepeda et al.'s meta-analysis pooled 839 spacing-effect tests across 317 experiments and 184 articles: the optimal interval between repetitions scales with the desired retention interval, at roughly 10–20% of it (i.e., to remember something a year out, review every 5–10 weeks) Cepeda et al. 2006. Expanding intervals beat fixed intervals; both beat massed practice. Bahrick et al. demonstrated this on the timescale that matters for language: 13 retraining sessions spaced at 56-day intervals produced retention 5 years later equivalent to 26 sessions at 14-day intervals — the wider spacing slowed initial acquisition slightly but doubled long-term efficiency Bahrick et al. 1993.

The testing effect. Roediger and Karpicke showed that students who studied a passage three times then took one recall test outperformed students who studied the passage four times (no testing) on a recall test one week later, with the test-group recalling roughly 50% more Roediger & Karpicke 2006. Karpicke and Roediger then showed the effect persists even when correct items are dropped from further study: repeated retrieval after first correct response drove long-term recall; repeated study after first correct response did not Karpicke & Roediger 2008. Active retrieval is doing computational work the brain treats as a storage strengthener; passive re-exposure is not.

Mechanistically, the two effects combine: each retrieval is harder when more time has passed, the retrieval itself strengthens the trace, and the next-due interval expands accordingly. The scheduler's job is to keep retrieval near the edge of failure where the storage strengthening is largest. SuperMemo's SM-2 algorithm (Wozniak 1987) was the first computational implementation; it powered Anki until 2023, when Anki adopted the FSRS (Free Spaced Repetition Scheduler) algorithm, which fits per-user parameters to recall history and on benchmark data needs 20–30% fewer reviews than SM-2 for the same retention target.

evidence

The cognitive-psychology evidence base is among the deepest in education research. Cepeda's meta-analysis (above) is the foundational quantitative synthesis. Bahrick's 50-year study tracked 733 individuals' retention of Spanish learned in high school: a substantial portion of vocabulary stays accessible for 25+ years without rehearsal in what he termed "permastore," and the strongest predictor was depth of original training, not rehearsal frequency during the retention interval Bahrick 1984. Pan and Rickard's meta-analytic review found retrieval practice produces an average transfer effect of d ≈ 0.40 vs non-practice controls across diverse final-test formats — meaningful but smaller than the within-format effect, with response congruency the strongest moderator Pan & Rickard 2018.

Real-world applications. Kerfoot et al.'s 2007 RCT randomised medical students to weekly spaced-education emails or no intervention: the intervention group's clinical-knowledge retention was significantly higher at follow-up Kerfoot et al. 2007. Deng et al. surveyed 72 Washington University medical students and found, after controlling for MCAT score, second-year honours, and test anxiety, that each additional 1,700 unique Anki flashcards reviewed was associated with one additional point on USMLE Step 1 — a clean dose-response signal in observational data Deng et al. 2015. Wothe et al.'s cohort study at the University of Minnesota found Anki users scored 6–13% higher across four medical school exams including the CBSE (a Step 1 proxy), with the benefit holding regardless of inherent test-taking ability Wothe et al. 2023.

Effect size translation. Across studies the rough numbers are consistent: spaced practice yields ~30–50% better retention than massed practice at long delays, retrieval practice yields ~20–50% better retention than re-reading at long delays, and the two compound. The effect sizes are unusually large for behavioural-science interventions and replicate across age groups, materials, and test formats.

protocol

A defensible default protocol for an adult using Anki:

New cards per day: 10–20 for a first language deck; 20–50 for full-time students (medical, law). 10 new cards/day = ~3,650 cards/year — roughly the high-frequency vocabulary of a foreign language.
Daily review session: 15–30 minutes is the sustainable band. Above 60 minutes/day, dropout rises sharply (community signal, not RCT data).
Algorithm: FSRS (default in Anki 23.10+, November 2023 onward). Set desired retention to 0.90 (90% of cards recalled on first try when due) for sustainable long-term use; 0.95 only when prepping for a high-stakes exam in <30 days.
Card design: One atomic fact per card. Avoid lists. Avoid "what are the three causes of X" (the card itself becomes the recall cue). Use cloze deletions for context-embedded facts. Add an image or sentence example when the bare term is ambiguous.
Source quality: For language, pre-made shared decks (e.g., frequency-ordered lists) are reasonable. For medical/professional, community decks (e.g., AnKing for USMLE) have years of refinement; rolling your own card-by-card is the high-quality but slow alternative.
Habit stacking: Pair with an existing daily anchor (morning coffee, commute, lunch). The streak-counter UI exists because consistency matters more than session length.

Empirical signal from medical school suggests early-implementation matters: cohorts who started Anki in first-year outperformed late starters even controlling for total card count Wothe et al. 2023.

contraindications

No medical contraindications — this is a software tool. Editorial cautions:

Subjects where understanding precedes memorisation. Pure mathematics, physics derivations, philosophical reasoning. SR can encode definitions and named theorems but does not substitute for working through proofs. Pan & Rickard's transfer literature shows the effect on far-transfer tasks is smaller than within-task Pan & Rickard 2018.
Anyone whose schedule cannot guarantee daily contact for months. Missing weeks turns the queue into a stress object; the failure mode is dropout rather than under-learning.
Compulsive personalities under exam pressure. Card-counting and streak-anxiety are real failure modes that can crowd out the deeper study they're supposed to support.

misconceptions

"More cards is more learning." Beyond the daily-time ceiling, additional new cards just compound future review load without proportionally improving retention. Anecdotal medical-school reports describe cohorts with 30,000+ cards reviewed who underperformed peers with 10,000 well-curated cards. The Deng dose-response is a population average, not a personal optimiser.

"Anki teaches you the language / the topic." Anki retains atomised facts. Conversational fluency, clinical judgement, and conceptual understanding require additional practice modalities (reading, speaking, problem-solving). SR is necessary-not-sufficient for any complex skill.

"I'll review when I have time." The scheduler's value is the scheduler. Skipping days creates a "review hell" backlog that triples ordinary daily load and is the leading driver of dropout (community reporting; not RCT-quantified).

"SM-2 is good enough." Was true for 35 years; less true now. FSRS measurably reduces reviews-per-retention-target; the upgrade was free and is the default in current Anki.

failure-modes

Card hoarding. Building enormous decks of low-value cards. Symptom: 200+ reviews/day, retention high, but nothing the learner cares about is in the deck.
Cramming-instead-of-spacing. Doing 500 reviews in one sitting twice a week instead of 70 reviews daily. Possible technically; defeats the algorithm's interval prediction.
Bad cards. Cards where the cue is too vague (e.g., "Spanish: door") or the answer is a list. Both produce a leech — a card that fails repeatedly and dominates session time. Anki flags leeches; the right move is to rewrite or delete.
Review-only, no encoding. Reviewing pre-made decks of facts the learner never genuinely studied first. Recall becomes recognition of the card's exact wording, not of the underlying knowledge.
Streak preservation under sleep debt. Doing reviews at 1 AM to keep the streak alive; poor retrieval-effort, poor consolidation, defeats the purpose.

practicalities

Anki: free on desktop (Windows/Mac/Linux), free on Android (AnkiDroid), $25 one-time on iOS (AnkiMobile). AnkiWeb sync is free. The most-used SRS by a large margin.
SuperMemo: the original, paid (€20–€50). Still uses Wozniak's proprietary newer algorithms (SM-18+); cult following, steeper UX learning curve.
RemNote, Mochi, Memrise, Quizlet: alternatives layering SR on notes, knowledge-management, or social features. Memrise and Quizlet have non-SR modes that crowd out the spacing benefit if used.
Time cost: at 15 minutes/day across a year, ~90 hours/year. At 30 minutes/day, ~180 hours/year. This is substantial; the comparison is to the (much larger) time that would otherwise be needed to relearn forgotten material.
Device: mobile is the high-frequency surface. A queue checked at the bus stop, in line, in waiting rooms is the difference between consistency and drift.

audience

Heaviest documented benefit in populations with large-stock atomised-fact learning loads: medical students, dental students, law students, language learners at intermediate-and-up levels, programmers maintaining knowledge of multiple languages or APIs, security and finance certification candidates. Casual learners (a hobbyist learning 50 Italian phrases for a vacation) get the spacing benefit but the overhead may exceed the gain — Duolingo-style apps that embed spacing in a lighter UX are usually a better fit at that scale.

alternatives

Alternatives that achieve some of SR's effect without the same overhead:

Light SR embedded in other apps — Duolingo, Babbel, Pimsleur all use some spacing. Lower ceiling, lower friction.
Self-quizzing during reading — pausing every paragraph to summarise from memory. The testing-effect benefit without the scheduler. Documented in Roediger & Karpicke and replicated extensively.
The Cornell note-taking method — left-margin cues plus active recall. A non-software way to operationalise retrieval practice.
Repeated past-paper exam practice — for high-stakes exams, doing 5–10 years of past papers under time pressure spaced over weeks is effectively spaced retrieval practice. Often beats card-based SR for application-heavy exams (e.g., math olympiads, bar exam).

The relative case for Anki: it is the most general-purpose, the most studied in clinical contexts, the cheapest, and the easiest to extend with custom content. The case against: it adds significant overhead (card-making, daily review) that is wasted if the underlying material is best learned through other modalities.

stakes

The reader who relies on memory in their work — clinician, lawyer, programmer, language teacher, translator — and does not use any retrieval-based system pays a quiet cost. Material learned for a one-time exam is forgotten over the next year following the Ebbinghaus curve; without rehearsal, only 20–30% of arbitrary detail remains at one year out and most of that is the most-used subset Murre & Dros 2015. The cost shows up as time spent re-looking things up, as quiet confidence-drop in domains the person used to feel fluent in, as the language they studied at university that they can no longer hold a conversation in. None of this is catastrophic; it is the slow erosion of expensive learning.

payoff

Multi-year SRS users report a different relationship with their learning: the fact that they actively studied two years ago can still be recalled now, the foreign language they last spoke fluently five years ago is still functional, the technical reading they did last quarter compounds rather than evaporates. The Bahrick permastore data suggests the deepest payoff appears at 5–25 years out — material that without SR would have been forgotten remains accessible Bahrick 1984, Bahrick et al. 1993. The reader's mental model shifts from "I have to relearn this every exam" to "I learn things once and keep them."

history

Ebbinghaus's 1885 monograph established the forgetting curve. The spacing effect was characterised in the 1960s–70s as the "lag effect." Piotr Wozniak began work on SuperMemo as a Polish student in 1985–87, releasing SM-2 — the first computational implementation — in 1987. Damien Elmes released Anki in 2006, open-sourcing the SM-2 derivative and making the technique accessible at zero cost. Medical-education research adopted SR in the mid-2000s (Kerfoot's spaced-education work; Deng's 2015 USMLE study Deng et al. 2015). The 2010s saw Anki become standard among medical students (community-built decks: AnKing, Lightyear, Zanki). In 2023 Jarrett Ye published the FSRS algorithm, machine-learning-fit to actual review data; Anki adopted it as default in November 2023. The field's still-active question is whether SR can be made to schedule conceptual material (not just facts) optimally.

out-of-scope

The article should signpost: language-learning tools (Duolingo et al.) for casual learners, broader study-skills frameworks (the testing effect as a study habit independent of software), and note-taking systems that pair with SR (Obsidian + Anki integrations, RemNote's combined note+card model).

The credibility range

The optimist case. Spaced repetition is one of the most robustly evidenced findings in cognitive psychology — Cepeda's meta-analysis covers 184 articles, the testing effect has been replicated hundreds of times across ages and materials, and the real-world translation is documented in medical school (Deng dose-response on USMLE) and clinical CME (Kerfoot RCT). The mechanism is biological (consolidation strengthens traces re-activated at the edge of forgetting), the algorithm is open-source and freely available, and the cost is roughly zero. Adopting SR is one of the highest-EV behavioural changes a knowledge worker can make. Multi-year users credibly report fluency in languages they would otherwise have lost, recall of clinical knowledge they would otherwise be looking up, and a different baseline confidence about what they "know."

The skeptic case. Most of the evidence is on narrow lab tasks (paired-associate learning) or on standardised exams that themselves test memorisation. Transfer to genuine application is smaller (Pan & Rickard d ≈ 0.40 vs much larger within-format effects). The medical-education observational studies are confounded by motivation — students who use Anki regularly are systematically the more conscientious students, and the dose-response could reflect grit rather than the algorithm. Dropout is high in casual contexts (precise rates are not RCT-quantified, but blog and community-survey signal is unanimous). For most consumer learning use-cases (50 phrases for a vacation, casual interest in a topic), the overhead of card-making and daily review exceeds the benefit. And spaced repetition can encourage learning the wrong things — cards memorised perfectly are no use if they are not the high-leverage facts the learner actually needs.

Author's call. The evidence for the spacing and testing effects is settled; FSRS is the current best-available implementation; the population in whom the practical case is strong is narrower than evangelists claim. For high-volume atomised-fact domains (medicine, law, languages at intermediate level, technical certifications), the case is overwhelming. For casual learning, a lighter tool is usually better. The honest framing for this entry: a high-evidence, high-effort tool that is transformative for the people who actually need it and a poor fit for those who don't.

Stakeholder + incentive map

Anki / Damien Elmes: developer of the dominant tool; non-commercial open-source project, sells iOS app for $25 to fund development. No advertising incentive.
SuperMemo World: commercial vendor of SuperMemo (€20–€50). Piotr Wozniak's writing (supermemo.guru) is a substantial part of the public discourse — pro-SR by definition, skeptical of competitors' algorithms.
Medical-education community: AnKing, Zanki, Lightyear deck maintainers; subreddit /r/medicalschoolanki (~250k members). Strong pro-SR consensus among USMLE-track students; mild establishment pushback from older faculty wary of "shortcut" framing.
Language-learning community: r/Anki, r/languagelearning, Refold method, Migaku — long-tail of bloggers and YouTubers promoting workflows.
Skeptic / counter-incentive: commercial language apps (Duolingo, Babbel) whose lighter-touch UX is differentiated against Anki's friction. Educational reformers wary of memorisation as displacement of understanding.
Cognitive scientists: Bjork lab (UCLA), Karpicke (Purdue), Pashler (UCSD), Cepeda (York). Broad consensus on the basic effects; ongoing debate on optimal scheduling, transfer, and curriculum integration.

Population variability

Age: spacing effect is universal but somewhat larger in children for vocabulary; older adults benefit at least as much as young adults at clinically meaningful effect sizes.
Domain dependence: larger effect on atomised-fact recall (vocabulary, anatomy, dates) than on conceptual / problem-solving tasks (Pan & Rickard moderator analyses Pan & Rickard 2018).
Personality: conscientious / high-trait-discipline users are over-represented in adoption and sustained use. Causal vs selection question is unresolved.
Baseline knowledge: Pan & Rickard find higher initial-test accuracy strongly moderates transfer — SR works better the more the learner already knew before they started carding it.
Language learners by stage: beginners benefit from light spacing inside lesson apps; intermediate-and-up learners benefit from dedicated SRS; advanced learners often drop the SRS in favour of native-content immersion.

Knowledge gaps

Specific gaps in the literature:

No large RCT of consumer-app Anki use (vs controlled spaced-education email interventions). The medical-school evidence is observational and confounded by selection.
Optimal interval for conceptual material vs declarative material is unsettled; some evidence suggests conceptual content benefits from longer intervals.
Dropout rates and the half-life of SRS habits are anecdotally well-known but not formally quantified at scale.
FSRS vs SM-2 head-to-head is benchmarked on log-loss against archived review data; no prospective RCT on learning outcomes.
Cross-cultural and L1-L2 typological-distance moderators are under-studied (most data is English-L1 learners).

Category choice. Filed under productivity. Considered language (too narrow — SR's medical-school evidence base is at least as strong as its language-learning one) and mental (more naturally houses cognition-as-substance entries like meditation). Productivity & Systems is the closest fit for a learning-system tool.

Body-axis scoring is mostly zero — that's correct. Spaced repetition is a cognitive tool, not a body intervention. The benefit dimensions in this catalogue are oriented to body / health / mind effects; SR's dominant effect (knowledge retention) is not one of them. The two non-zero benefit scores stretch their dimension definitions modestly:

focus = 2: the cognitive lift is knowledge-dependent (retained material enables performance in cognitive tasks that need it) rather than a direct attention boost. Same logic underwrites the example entry's creatine focus = 2 ("modest improvements in cognitive performance"). The Deng Deng 2015 and Wothe Wothe 2023 exam-score data anchor the call.
mood = 1: conservative; no clinical-mood literature on SR. Kept non-zero to honour the topic brief's explicit naming of "recall confidence" as a consequence — the felt experience of competence in what you know is a real (if small) wellbeing contribution.

Skipped sections. No stakes: quiet erosion of learning doesn't anchor on a typical reader the way stakes for sleep-debt or alcohol does, and forcing a stakes section would have read as fear-mongering for a non-health topic. Payoff carries the future-state contrast on its own. No practicalities — device/cost/time folded into protocol and alternatives. No history — Ebbinghaus → Wozniak → Anki → FSRS lineage is in the research dossier; the article surfaces only the load-bearing parts (Ebbinghaus in mechanism, FSRS in protocol). No audience section — heaviest documented benefit is in high-volume-fact populations (med students, intermediate-and-up language learners) but the article body addresses this throughout rather than carving out a separate block.

Brief coverage. The four consequences the brief named all have a home: long-term retention in mechanism + evidence + payoff; study-time efficiency in mechanism (5 reviews vs 50 re-reads; Bahrick's 13 vs 26 sessions); recall confidence in payoff and the mood pitch; learning consistency in protocol + failure-modes.

Dropout / "review hell" caveat. The article reports the failure mode as the leading driver of quitting; the dossier flags this as community-signal rather than RCT-quantified. No peer-reviewed dropout-rate study was found for consumer Anki use.

Future-link candidates (not yet entries).

Retrieval practice as a habit — the testing effect operationalised without software (Cornell notes, recall-while-reading, past-paper practice). Separate from SR-the-tool.
Language-learning apps (Duolingo, Babbel, Pimsleur) — the lighter-touch alternative for casual learners; alternatives section signposts but doesn't compare.
Learning a new language as an adult — broader entry that would naturally cross-link here for the vocabulary-storage layer.
FSRS vs SM-2 deep-dive — likely not its own entry; algorithm-internals are too narrow.

Separate-entry candidates surfaced during research. AnKing / Zanki specific medical-school decks have substantial community literature but probably belong as workflow notes inside a hypothetical USMLE-prep entry rather than as standalone catalogue items.

Audience field. Left unset (applies to everyone). Considered restricting to 18-39 + 40-59 (peak high-volume-learning years) but adult language learners and clinicians span all three age bands; over-scoping would have shrunk reach unnecessarily.