Forgetting a new word days after you learn it is frustrating. You study a list, feel confident, and then draw a blank when you need the term. The problem isn't your memory, it's your review method. Spaced repetition is a powerful technique designed to solve this exact problem.

Spaced Repetition, or Spaced Repetition System (SRS), is a memorization technique that involves reviewing information at increasing intervals. Instead of cramming, you review a word just as you're about to forget it, strengthening its place in your long-term memory. It's the most efficient way to learn and retain vocabulary, and apps can automate this entire process for you.

How Spaced Repetition Fights the "Forgetting Curve"

In the 19th century, psychologist Hermann Ebbinghaus discovered the "forgetting curve." It shows that we forget new information exponentially unless we actively review it. Within 24 hours of learning something new, most people retain only about 50% of the material. After a week without review, that number drops to around 10%.

Spaced repetition directly counters this by scheduling reviews at specific moments:

  1. Initial Learning: You encounter a new word and its definition.
  2. First Review: You review it soon after, perhaps within a day.
  3. Increasing Intervals: Each time you correctly recall the word, the time until the next review gets longer (e.g., 2 days, then 4 days, then 8 days, and so on).
  4. Incorrect Recall: If you forget the word, the interval shortens, bringing it back for review sooner.

This system focuses your effort on the words that are hardest for you to remember, saving you time from studying what you already know.

A Simple Spaced Repetition Schedule for Flashcards

While a dedicated app is the easiest way to manage this, you can try a basic version manually with physical or digital flashcards. The key is to separate your cards into different review piles or boxes based on how well you know them.

Here is a popular, simplified schedule known as the Leitner System:

  • Step 1: Create Your Flashcards Write the word on one side and the definition on the other. For example, Ephemeral on the front and "Lasting for a very short time" on the back.

  • Step 2: Daily Review (Box 1) All new cards start in Box 1. Review this box every day. If you get a card right, move it to Box 2. If you get it wrong, it stays in Box 1.

  • Step 3: Extended Review (Boxes 2, 3, 4...)

    • Box 2: Review every 2-3 days. If correct, move it to Box 3. If wrong, move it back to Box 1.
    • Box 3: Review once a week. If correct, move it to Box 4. If wrong, move it back to Box 1.
    • Box 4: Review every two weeks, and so on.

Manually tracking this can be tedious, which is why Flashi's AI-powered flashcards are so effective. The app automatically schedules your daily "Word of the Day" and any custom flashcard sets you create, ensuring you review each term at the optimal time for long-term retention without any of the manual sorting.

How SRS Algorithms Work Under the Hood

Not all spaced repetition apps use the same algorithm. Understanding the differences helps you choose the right tool.

SM-2 (SuperMemo 2)

The SM-2 algorithm is one of the oldest and most widely used SRS systems, developed by Piotr Wozniak in 1987. It calculates a card's next review date based on two factors: how many times you've gotten it right in a row (repetitions) and an "ease factor" that reflects how difficult the card is for you personally. Cards you consistently answer correctly get progressively longer intervals. Cards you struggle with get shorter intervals and reset closer to the beginning of the schedule.

SM-2 is reliable and battle-tested. Many popular flashcard apps, including early versions of Anki, have been built on this foundation.

FSRS (Free Spaced Repetition Scheduler)

FSRS is a newer algorithm that uses a machine-learning approach to model human memory more accurately. Rather than relying on simple repetition counts and a fixed ease factor, FSRS builds a detailed model of your retrievability for each card over time. It takes into account how long ago you last reviewed something and how well you did, then predicts the exact moment your memory of that card is likely to drop below a target threshold.

In practice, FSRS tends to assign longer intervals on easy cards (reducing unnecessary reviews) and shorter, more targeted intervals on difficult cards. Studies comparing FSRS to SM-2 have shown improved retention with fewer total review sessions.

Flashi's review system is designed around the principle that reviews should happen at the right time, not just on a fixed daily schedule, aligning with this memory-science approach.

A Comparison of the Two

Feature SM-2 FSRS
Algorithm basis Rule-based intervals Predictive memory modeling
Ease factor adjustment Manual, per card Automated based on performance history
Interval length on easy cards Can be shorter than optimal Generally longer, reducing over-review
Best suited for Any vocabulary learner Learners who want maximum efficiency
Setup complexity Simple Slightly more configuration in some apps

For most vocabulary learners, either algorithm will produce meaningful results. The biggest gain comes not from choosing between SM-2 and FSRS but from using any spaced repetition system consistently versus not using one at all.

Why SRS is Ideal for Vocabulary Learning

Spaced repetition is perfect for vocabulary because it treats each word as a separate piece of information with its own difficulty and review schedule. This is very different from re-reading a chapter, which gives every piece of information equal attention regardless of whether you know it already.

Whether you're studying for the SAT, learning Japanese, preparing for medical boards, or just building a richer daily vocabulary, an SRS ensures that:

  • Words you already know well take up very little of your time.
  • Words you're struggling with appear frequently until they stick.
  • The total daily review load stays manageable, usually under 10 minutes, even as your deck grows to hundreds of cards.
  • You build a genuine long-term memory for each word, not just short-term familiarity.

Vocabulary Types That Benefit Most from SRS

Spaced repetition works well for any discrete fact you need to memorize. But some vocabulary categories benefit more than others.

High benefit:

  • Foreign language vocabulary (any language)
  • Medical and legal terminology (where precision matters)
  • Standardized test vocabulary (GRE, SAT, TOEFL, IELTS, NCLEX)
  • Technical jargon in a new field

Lower benefit (but still useful):

  • Idioms and collocations that depend heavily on context
  • Grammar rules that require application practice, not just recall

For the high-benefit categories, daily SRS practice is one of the highest-ROI study activities you can do. Five minutes a day compounded over three months produces dramatically better results than occasional long study sessions.

Getting Started with a Spaced Repetition App

The simplest way to use spaced repetition for vocabulary is to start with a dedicated app. Flashi is designed for exactly this purpose. It handles the scheduling automatically so you don't have to track any intervals yourself.

Here's how to begin:

  1. Create a deck for your current vocabulary focus (e.g., "GRE High-Frequency Words" or "Japanese N4 Verbs").
  2. Add your first 20-30 words using the AI import feature: paste a list and let the app generate the definitions.
  3. Review daily, even if just for five minutes. The app shows you the cards that are due based on your performance history.
  4. Add new cards gradually as you exhaust your current deck. Adding too many at once increases the daily review load faster than you can handle.

The goal isn't to finish a deck quickly. The goal is to build a review habit that compounds over months, gradually shifting more and more words into long-term memory.

Download Flashi and start your first spaced repetition deck today. It's free on iPhone and requires no account.

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