Spaced Repetition
The science behind how LanguageDen schedules your reviews.
What is spaced repetition?
Spaced repetition is a study technique that schedules reviews at increasing intervals. Instead of reviewing every word every day, you review each word just before you are likely to forget it. This exploits the spacing effect — the well-established finding that memory is strengthened more by spreading reviews over time than by massing them together.
The result is that you spend less total study time per word while retaining it for much longer.
How LanguageDen schedules reviews
LanguageDen uses FSRS (Free Spaced Repetition Scheduler) — a modern open-source algorithm that models how memories fade and recovers. Unlike older SM-2-style schedulers, FSRS does not use an ease factor. Each Word carries two memory parameters:
- Stability — how many days the algorithm expects you to still recall the word. Higher stability means longer gaps between reviews.
- Difficulty — how hard the word is for you personally. Harder words grow stability more slowly.
FSRS targets a desired retention of 90%: it aims to surface each word again when your chance of recalling it has fallen to about 90%. That keeps reviews efficient without letting forgetting slip too far.
Scheduling lives on the Word, not on individual flashcards. When you rate a card during a session, LanguageDen updates that word's FSRS state and next review time. FlashcardSelector then chooses which of the word's flashcards to show on the next review — based on learning stage and part of speech — so you practice the word from different angles over time.
Word states
Every word moves through a small set of FSRS states:
| State | Meaning |
|---|---|
| New | Never reviewed. No stability or difficulty yet. |
| Learning | Early reviews while the word is still consolidating into memory. |
| Review | Graduated into the long-term schedule with growing intervals. |
| Relearning | Failed a review (Again) after being in Review — short intervals until it recovers. |
Typical flow is New → Learning → Review. An Again rating in Review moves the word to Relearning; successful recalls move it back to Review.
How ratings affect scheduling
After you reveal the answer, you rate the recall with one of four buttons. FSRS uses the rating to update stability and difficulty, then computes the next interval:
Exact intervals depend on the word's current stability, difficulty, and how long it has been since the last review — not on a fixed multiplier. During a session, the rating buttons preview the next interval FSRS would assign for each choice. Other docs pages link here so the rating explanation stays in one place.
Why words come back even when you know them
If you rate a word Easy consistently, its stability grows quickly. You might see it today, then in a few days, then weeks, then months. That is FSRS working correctly — pushing the review out toward your predicted forgetting point while holding ~90% retention.
If a word keeps returning frequently, you are usually rating it Hard or Again, which keeps difficulty higher and stability lower until recall is solid.
Retention and accuracy
FSRS itself aims for about 90% retention when scheduling. On the Stats page, accuracy is a related but separate number: the percentage of your last 100 reviews that were not rated Again (so Hard, Good, and Easy all count as successful recalls).
Accuracy well above 90% can mean you are adding words slowly or rating conservatively. Accuracy much below 80% often means new words are outpacing consolidation — slow the intake or rate more honestly.
Suspended and flagged words
Suspended words are excluded from the review queue entirely. Flagging a word does not affect scheduling — it simply marks the word for your own reference. See Word Filters for how to manage suspended and flagged words.