You have probably heard that reviewing vocabulary on a schedule is better than random review. The reason goes deeper than habit: the timing of review has a measurable, predictable effect on how well information is retained in long-term memory. Spaced repetition formalizes this into a system.
What Is Spaced Repetition?
Spaced repetition is a review scheduling method based on the spacing effect — a cognitive phenomenon documented by Hermann Ebbinghaus in the 1880s and extensively validated by researchers ever since. The core principle: reviewing an item shortly before you are about to forget it produces better long-term retention than reviewing it frequently when it is still fresh.
The Forgetting Curve
Ebbinghaus showed that without any review, humans forget approximately 70% of newly learned information within 24 hours. By day seven, retention drops to around 20% for most types of factual knowledge. The key insight is that each successful recall resets the curve and extends the interval before the next review is needed. The curve is slower to decay after each review — meaning that items you have reviewed three times can go weeks before the next review, while items reviewed once need to come back the next day.
The SM-2 Algorithm
The most widely used algorithm behind spaced repetition systems (including Anki and many language apps) is the SM-2 algorithm, developed by Piotr Wozniak in the 1980s. SM-2 assigns each item an "ease factor" based on how difficult you find it, and uses this to calculate the next review interval. Items you know confidently get longer intervals; items you struggle with come back sooner.
Why Spaced Repetition Is Particularly Powerful for Japanese
Japanese has one of the largest vocabulary requirements of any language commonly studied by English speakers. N1 proficiency requires approximately 10,000 vocabulary items, N2 around 6,000, N3 around 3,750. Without a system to manage when you review each of these thousands of words, you will either over-review items you know well (wasting time) or under-review items you need (losing them to forgetting). Spaced repetition solves both problems simultaneously.
- It focuses your study time on items at risk of being forgotten
- It avoids wasting time on items you already know solidly
- It gives you a measurable, growing collection of truly mastered vocabulary
- It scales with your level — the more you know, the more the system handles efficiently
How to Use Spaced Repetition Effectively
The biggest mistake people make with spaced repetition systems is treating them as passive review tools. You see the front of the card, think "I know that," flip it, and say "yes, correct." This does not constitute active recall. For spaced repetition to work at maximum effectiveness, you need to actually retrieve the answer before you see it.
The Correct Way to Review a Card
When a card comes up, cover or hide the answer. Actively try to recall the information — say it aloud or type it out. Only then reveal the answer. Rate your performance honestly: did you recall it immediately and confidently? Did you hesitate? Did you get it wrong? Your rating determines the next interval, so accuracy here determines the quality of the schedule.
Daily Review Discipline
The power of spaced repetition depends on not skipping review days. Each item is scheduled to appear at its optimal review interval. If you skip a day, those items are due regardless when you return, and a backlog of overdue items can be demotivating and inefficient. Consistency — even just 15 minutes per day — is more important than longer, infrequent sessions.
New Cards vs. Review Cards
A common mistake is adding too many new cards too quickly. Your daily review load is the sum of all cards in the system that are due — if you add 50 new words per day for a week, you will have hundreds of review cards per day within a month. A sustainable pace for most learners is 10–20 new items per day, resulting in a manageable review load of 100–200 cards per day at steady state.
Spaced Repetition vs. Cramming
Research comparing spaced repetition to massed practice (cramming) consistently shows a dramatic advantage for spaced review, especially for retention measured after one week or more. A meta-analysis by Cepeda et al. (2006) covering 254 studies found that spacing review sessions produced 100–200% better retention than massing the same amount of study time. For Japanese learners preparing for JLPT, this means starting vocabulary review months before the exam date — not in the final week.
Combining Spaced Repetition with Active Recall
The most effective approach combines both techniques: use a spaced repetition system to determine which items to review, and use active recall as the method of review. This means: when the system shows you a word, type the Japanese translation from memory before revealing the answer. The spaced schedule ensures you review at the right time; active recall ensures the review is actually doing productive work. This combination is the core of evidence-based vocabulary study for Japanese.
Getting Started
If you are new to spaced repetition, start small. Add only 10 new items per day for the first two weeks, and focus on maintaining a consistent daily review habit. Once the habit is established, you can gradually increase the pace. The most important thing is not how many cards you add — it is that you show up every day to review the items that are scheduled.