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Spaced repetition

In Introduction we looked, intuitively, at the forgetting curve and at why "reviewing at the right time" works. Let's take the same idea a bit deeper here.

The relationship between the forgetting curve and the spacing effect​

When you first learn something, forgetting starts fast. But something interesting happens after each successful recall: the next round of forgetting progresses much more slowly. In other words, if you successfully recall a piece of information right as you're about to forget it, its durability increases compared to before, and the interval before the next review can stretch out further.

This is called the spacing effect: the finding, backed by decades of psychology research, that spreading the same total number of reviews out over time produces markedly better retention than cramming them together back-to-back.

What does a scheduling algorithm do?​

Here's what makes this impossible to track by hand: for dozens or hundreds of different cards, the answer to "when is this card about to be forgotten" is completely different every time. A card you find easy might stick in your memory for months, while one you find hard can slip away within a few days.

This is exactly what a scheduling algorithm's job is: to answer the question "when should the next review happen" separately for every single card. The general logic works like this:

  • When you successfully recall a card, the interval before its next review grows (by more or less, depending on how easily you recalled it).
  • When you forget a card, that interval shrinks: the card comes back around soon.

Over time, the algorithm learns each card's own "forgetting rate" and personalizes the schedule accordingly.

FSRS in Tsunagi​

For scheduling, Tsunagi uses FSRS (Free Spaced Repetition Scheduler), a modern, multi-parameter algorithm. We won't go into FSRS's inner workings here. You don't need to know them for everyday use, the same way you don't need to understand how GPS satellites work to use a map. For anyone curious, this could be the subject of a separate, optional article down the line.

All you need to know is this: Tsunagi tracks every card individually and uses this logic to decide when to show you which card.

Spaced repetition tells us when we need to review. But how we work during that review matters just as much: that's what we'll look at next.