What spaced repetition is (and why rereading fails)

You read the chapter three times. You highlighted everything. In the exam, blank.

It isn't lack of effort. Rereading is one of the least efficient ways to study — and there's a better one, with nearly fifty years of research behind it.

The forgetting curve

In 1885, Hermann Ebbinghaus ran an experiment on himself: he memorised lists of nonsense syllables and measured how much he retained over days. The result became known as the forgetting curve, and it's brutal — most of what we learn slips away in the first few days, and what survives drifts down to almost nothing.

Rereading doesn't bend that curve. It creates a feeling of mastery — the text looks familiar, so the brain concludes it knows. Familiarity isn't memory.

What does bend it

Two things, and both are uncomfortable:

• Active recall: trying to remember without looking. The effort of pulling the information out is what strengthens the path to it.
• Spacing: reviewing at growing intervals, right when you're about to forget.

A review on a memory that's still fresh is nearly useless. A review on a memory that's already gone is learning from scratch. The sweet spot sits between the two — and that's exactly where a spaced repetition system tries to place you.

Each well-timed review flattens the drop and pushes the next one further out. A card that comes back in three days today will come back in three weeks a month from now.

Where SM-2 comes in

SM-2 is the algorithm Piotr Woźniak created in the 1980s for SuperMemo, and it underpins much of the flashcard world to this day — including Anki for many years. It tracks three things per card:

1. Interval — how many days until the next review
2. Repetitions — how many times in a row you got it right
3. Ease factor (EF) — how easy this card is for you

Say a card was easy and the factor rises, stretching the interval. Get it wrong and the interval collapses, sending the card back to the front of the queue. That's why your honest rating matters more than your score: pretending it was easy only postpones the problem to exam week.

What this means in practice

Spaced repetition isn't a memorisation trick. It's a trade: you accept the bad feeling of forgetting and getting things wrong today, in exchange for remembering six months from now.

It also imposes a boring discipline: it has to be daily. A spaced system only knows what to show you today if you showed up yesterday. Twenty minutes a day beats six hours the night before — not because six hours is little, but because the curve doesn't forgive.

The best time to review is just before you forget. The problem is you don't know when that is. The algorithm does.

Where to start

Pick one subject. Make twenty cards. Answer them every day for two weeks and look at your accuracy rate — the number, not the feeling.

If the thought of typing card after card already put you off, that's precisely where most people quit. It's why Mindstreak generates cards with AI from a topic, and imports the decks you already have in Anki: the value is in reviewing, not in formatting.