The Science of Spaced Repetition

Spaced repetition is one of the most thoroughly researched learning techniques. It works, and we understand why it works. Here's the science behind it and how to apply it to language learning.

The Core Insight

Memory isn't permanent by default. When you learn something, it begins fading immediately. Without reinforcement, most new information is forgotten within days.

But each time you successfully retrieve information from memory, you strengthen it. The memory becomes more durable. It fades more slowly. With enough retrievals at the right intervals, information can be retained essentially permanently.

Spacing Effect

The "spacing effect" was discovered in the 1880s by Hermann Ebbinghaus. He found that distributed practice (reviewing at intervals) produces better retention than massed practice (cramming) for the same total study time.

Why? Each retrieval does two things:

1. Strengthens the memory trace
2. Resets the forgetting curve at a slower decay rate

After enough successful retrievals, the memory becomes so stable that it persists for months or years without review.

Testing Effect

Related research revealed the "testing effect": retrieving information from memory (testing yourself) strengthens learning more than simply re-studying the same material.

This is why flashcards work better than re-reading. The act of trying to recall forces deep processing. Passive review doesn't have the same effect.

Optimal Intervals

Research suggests reviewing information just as you're about to forget it maximizes efficiency. Too soon, and you're wasting time on material you still know well. Too late, and you've forgotten and must relearn.

Modern spaced repetition algorithms (like those in Anki) use your performance to estimate when you're about to forget and schedule reviews accordingly. This personalization makes the system more efficient than fixed schedules.

Application to Language Learning

Vocabulary

Vocabulary is ideal for SRS. Thousands of discrete items need to be memorized. Without spaced repetition, you'd constantly relearn forgotten words. With it, you review efficiently and retain long-term.

Grammar

Grammar rules and example sentences can be SRS items too. Create cards for conjugation patterns, exception cases, or confusing structures.

Sentences

Sentence cards combine vocabulary and grammar. You see a sentence and must understand or translate it. This tests words in context, which is closer to real-world use.

How Algorithms Work

Popular algorithms like SM-2 (used in Anki) work roughly like this:

• New cards start with short intervals (1 day, then a few days)
• Successful recall increases the interval (exponentially at first, then more gradually)
• Failed recall resets or significantly reduces the interval
• Cards you know well eventually have intervals of months

The result: you spend most review time on items you find difficult, and minimal time on items you know well.

Practical Considerations

Daily Reviews

SRS works best with daily reviews. Skipping days causes cards to pile up. The algorithm assumes you'll review on time; delays create backlogs and can make cards harder than expected.

Card Quality

Poorly designed cards undermine the system. A card that's too complex (testing multiple things at once) or ambiguous (multiple possible correct answers) will fail repeatedly, not because you don't know the material but because the card is bad.

Adding New Cards

New cards create future review obligations. Adding too many at once creates overwhelming review loads later. Sustainable addition rates depend on your available time.

Limitations

SRS is powerful but not complete:

• It tests recognition more than production
• It doesn't teach meaning or usage deeply
• It can feel mechanical and boring
• It requires discipline to maintain

SRS is one tool among many. It solves the forgetting problem but doesn't replace reading, listening, speaking, and writing practice.

Getting Started

To use spaced repetition for language learning:

1. Choose an app (Anki is free and powerful; others like Memrise exist)
2. Start with a pre-made deck or create your own cards
3. Add new cards slowly (10-20 per day maximum to start)
4. Review every day
5. Trust the algorithm—if cards keep appearing, you keep forgetting them

The science is clear: spaced repetition works. Consistent use can help you retain thousands of vocabulary items that would otherwise be forgotten. It's one of the highest-leverage techniques available to language learners.