The Science of Spaced Repetition: Why It Works

What is Spaced Repetition?

Spaced repetition is a learning technique that involves reviewing information at gradually increasing intervals. Instead of cramming everything in one session, you space out your reviews over time—first after a day, then a few days, then a week, and so on.

This method is based on the forgetting curve, discovered by German psychologist Hermann Ebbinghaus in 1885. His research showed that we forget approximately 70% of new information within 24 hours unless we actively review it.

The Science Behind It

The Forgetting Curve

Ebbinghaus's experiments revealed that memory decay is exponential. Within the first hour of learning something new, we forget about 50% of it. By the end of the day, that number rises to 70%. After a week, we might only retain 20% of the original information.

But here's the fascinating part: each time we review the information, the forgetting curve flattens. The memory becomes more stable and lasts longer before needing another review.

How Spaced Repetition Exploits This

The key insight is that the optimal time to review something is right before you're about to forget it. Review too early, and you're wasting time on something you already know. Review too late, and you've already forgotten it—you're essentially relearning from scratch.

Spaced repetition algorithms (like the SM-2 algorithm used by many flashcard apps) calculate the optimal review time for each piece of information based on:

1. How difficult the item is - Harder items need more frequent reviews

• Your performance history - Items you consistently get right can be spaced further apart
  • Time since last review - The interval increases with each successful recall

    Why It's So Effective

    1. Strengthens Neural Pathways

    Every time you successfully recall information, you strengthen the neural pathways associated with that memory. It's like walking the same path through a forest—the more you walk it, the clearer and more permanent it becomes.

    2. Desirable Difficulty

    Psychologists Robert Bjork and Elizabeth Bjork introduced the concept of "desirable difficulties"—challenges that slow down learning initially but lead to better long-term retention. The effort of recalling information just before you forget it is one such difficulty.

    3. Efficient Use of Time

    By focusing your study time on the items you're about to forget, you're not wasting time on things you already know well. This can reduce study time by 50% or more compared to traditional methods.

    4. Prevents Illusion of Competence

    When you re-read notes or textbooks, you might feel like you know the material because it seems familiar. But recognition is not the same as recall. Spaced repetition forces active recall, revealing what you truly know versus what you only recognize.

    Research Evidence

    A landmark 2006 study by Rohrer and Taylor found that students who used spaced practice scored 64% on a test, compared to 32% for those who used massed practice (cramming)—a 100% improvement.

    A 2008 study by Karpicke and Roediger showed that repeated retrieval through spaced repetition was more effective than concept mapping or other elaborative study strategies.

    Meta-analyses consistently show that spaced repetition produces effect sizes of 0.5 to 1.0, which translates to moving from the 50th percentile to the 69th-84th percentile.

    How to Apply It

    1. Use a Spaced Repetition System (SRS)

    Apps like Explanor automatically calculate optimal review intervals for you. You don't need to manually track when to review each card.

    2. Keep Cards Simple

    Each flashcard should test one piece of information. Complex cards are harder to schedule and often lead to partial recall.

    3. Be Consistent

    The system works best when you review daily. Even 15 minutes a day is more effective than 2 hours once a week.

    4. Trust the Algorithm

    Resist the urge to skip "easy" cards or cram before exams. The algorithm knows when you need to review something—trust it.

    The Bottom Line

    Spaced repetition isn't just another study technique—it's arguably the most evidence-backed method for long-term memory formation. By reviewing information at optimal intervals, you can learn more in less time and actually remember what you've learned.

    The best part? Modern tools like Explanor handle all the scheduling for you. All you need to do is show up and review.