Spaced repetition and the forgetting curve

What the forgetting curve describes

In the 1880s, Hermann Ebbinghaus tested his own memory with nonsense syllables and plotted how quickly he forgot them. Recall dropped sharply in the first day, then more slowly over the following weeks. That pattern — steep early loss, long tail — is the forgetting curve.

Modern work often models recall as decay over time. A simple exponential form is (R(t) = e^{-t/S}), where (t) is time since you last successfully recalled something and (S) is a stability (memory strength) measured in days. Higher stability means the same interval produces higher recall.

You cannot eliminate forgetting. What you can do is interrupt the curve by reviewing just before recall would drop too far.

Why cramming fails

Cramming pushes many exposures into a short window. That can feel like mastery on test day, but without spaced reviews the curve wins: most of the material is gone within a week unless you keep using it.

Spaced repetition spreads reviews across days and weeks. Each successful recall strengthens the memory and pushes the next review further out. The goal is not more total study time — it is better-timed study time.

How spacing helps

Each time you retrieve an answer from memory (not merely re-read it), you strengthen the path to that fact. Spaced repetition schedules those retrievals:

1. Soon after learning — while the memory is still fragile.
2. Before you would forget — so retrieval is effortful but usually successful.
3. With expanding gaps — successful reviews justify longer intervals.

HippoCampus uses a modified SM-2 scheduler (the same family as classic SuperMemo) with global constants tuned for the product. After you grade a card, the app sets the next due date from your rating (Again, Hard, Good, Easy) and the card’s current interval and ease.

What “success” means in the app

A review counts as successful when you answer in your own words and the grader marks the response as correct enough to move forward. Failed or partial answers shorten the interval or send the card back into relearning, depending on the rating you choose.

Over time, well-learned cards may sit for weeks or months between reviews. Cards you struggle with come back sooner. That is the practical effect of fighting the forgetting curve: more time on what is still weak, less time on what is already stable.

Measuring your own curve

HippoCampus stores every review with a timestamp and rating. From that history the app can estimate how recall varies with elapsed time since the last review — a personal retention curve on the Statistics page. It is a readout, not a promise: use it to see whether your intervals feel too aggressive or too lax.

Takeaways

• Forgetting is normal; the curve is steepest right after learning.
• Spaced repetition schedules reviews when they do the most good.
• Active recall (answering, not rereading) drives long-term retention.
• An algorithm can adjust intervals, but you still choose honest ratings after each card.