Spaced Repetition and Memory

Hermann Ebbinghaus discovered forgetting in 1885. He memorized nonsense syllables, tested himself at intervals, and plotted the decay. The result — the forgetting curve — shows memory dropping exponentially: most of what we learn is lost within days without review.

But Ebbinghaus also discovered that each review slows the decay. The curve flattens. Memories that seemed fragile become durable. The key is when you review: too soon and you waste effort on material you still know; too late and you’ve forgotten completely. There’s an optimal interval — just before the memory fades.

This is spaced repetition: reviewing material at expanding intervals to maximize retention with minimum effort.

Retrieval practice — actively recalling information — is more effective than passive review. Rereading a textbook feels like learning but produces weak retention. Trying to answer a question from memory, then checking the answer, produces strong retention.

The effort of retrieval is the signal. The struggle to remember is where learning happens. If retrieval is easy, you’re not learning much. If retrieval fails completely, you need the information closer to hand. Optimal learning lives in the zone of productive difficulty.

This is the testing effect: being tested on material produces better long-term retention than spending the same time studying. Testing isn’t just assessment — it’s the learning.

Modern spaced repetition software (Anki, SuperMemo, Mnemosyne) automates the scheduling. You create flashcards; the software presents them at optimal intervals based on your performance. Cards you answer correctly get scheduled further out; cards you miss come back sooner.

The algorithm (SM-2 or variants) is based on research by Piotr Wozniak, who spent decades studying optimal review schedules. The math is complex; the practice is simple. Answer cards daily. Rate your recall. The system handles the rest.

Consistency matters more than volume. Ten cards per day, reviewed every day, compounds dramatically. Hundred cards once a week loses to ten cards daily. Spaced repetition is a daily practice, like exercise or meditation.

What to put on cards:

Factual knowledge that needs to be instant: medical terminology, programming syntax, vocabulary, historical dates, legal standards. Material where slow recall is as useless as no recall.

Conceptual hooks that trigger deeper understanding: the card prompts recall of a concept you understand; you’re maintaining access, not learning the concept from the card.

Material with no natural context for recall: your daily life won’t spontaneously quiz you on the capitals of Southeast Asian countries. Cards provide the context.

What not to put on cards:

Complex conceptual understanding: flashcards work for atoms of knowledge, not essays. You can card “What is the forgetting curve?” not “Explain the theory of spaced repetition.”

Skills: you can’t flashcard your way to speaking a language or playing an instrument. Cards support skills by maintaining vocabulary or factual scaffolding; they don’t replace practice.

Material without context or meaning: if you don’t understand why something matters, memorizing it is fragile. Comprehension first, cards second.

The spacing effect extends beyond flashcards. Interleaved practice — mixing topics within a study session — beats blocked practice (focusing on one topic at a time). The interruption forces retrieval, which strengthens memory.

Desirable difficulties — effortful processing that feels harder but produces more durable learning — include spaced practice, interleaving, varying conditions, and testing over rereading. The common thread: easy learning is shallow learning.

Anki is the dominant tool. It’s free, open-source, cross-platform, with a mobile app. The learning curve is real — effective card creation requires practice. Bad cards (vague, ambiguous, too complex) produce bad results.

Effective card design:

One fact per card. Atomic knowledge.
Clear, unambiguous questions. The answer should be specific.
Cloze deletions for connected material. “The forgetting curve was discovered by {{c1::Ebbinghaus}} in {{c2::1885}}.”
Images and examples for context. Visual memory is powerful.

The daily review session takes 10-30 minutes for a mature deck. This small investment maintains access to thousands of facts indefinitely.

Spaced repetition is maintenance. It doesn’t replace understanding or create expertise. It maintains the factual scaffolding that expertise requires. The surgeon who can recall anatomy instantly frees cognitive resources for judgment; the one who must look things up is slower and more error-prone.

Memory isn’t the goal. Access is the goal. Spaced repetition is the most efficient known method for maintaining access over time.

Go Deeper

Books

Make It Stick by Brown, Roediger, and McDaniel — Cognitive science of learning, including spacing and testing effects.
Fluent Forever by Gabriel Wyner — Spaced repetition applied to language learning.

Essays

Michael Nielsen’s “Augmenting Long-term Memory” — Deep dive on personal Anki use for serious learning.
Gwern’s “Spaced Repetition for Efficient Learning” — Comprehensive review of research and practice.

Tools

Anki — The standard. Requires investment to learn well.
RemNote, Mochi — Alternatives with more integrated note-taking.