Spaced repetition for vocabulary: the intervals that actually work

Spaced repetition is the one study technique with a century of evidence behind it, and it is embarrassingly simple: review something right before you would forget it, and each time you succeed, wait longer before the next review.

The idea in one paragraph

Memory decays on a curve. Immediately after you learn something the curve is steep; you forget fast. Each time you successfully recall the item, the curve flattens. So the efficient time to review is just before the point where recall would fail, and that point moves further out every time. Reviewing sooner wastes effort. Reviewing later means relearning from scratch.

Why rereading feels better and works worse

Rereading a word list is smooth and comfortable, and it produces a strong feeling of familiarity. That feeling is the problem. Familiarity is recognition, and recognition is not what you need when you want to use a word in a sentence. Active recall is harder and less pleasant, and it is the thing that builds the memory. The mild difficulty is not a bug.

The intervals

There is no magic schedule, but there is a shape that works. WordVale uses this ladder:

Get a word right and it climbs one rung. Miss it and it drops back, so it returns sooner. After the sixth rung a word is considered mastered and mostly leaves the queue. Six successful recalls over about five months is what it takes to own a word for good, which is far less than most people assume.

What most people get wrong

• Too many new words at once. Every new word adds future reviews. Ten a day is plenty; thirty a day buries you by week three.
• Grading yourself generously. If you had to think hard, that is a miss. Be honest and the schedule stays accurate.
• Skipping days. The queue does not pause. Miss a week and you face a wall. The fix is a small daily goal you can hit on a bad day, and a rule that a "freeze" is allowed once in a while so one missed day does not break the streak.
• Cards with no context. A word and a one-line definition is the weakest possible card. The sentence you met the word in is the strongest.

How long each day

With ten new words a day and the ladder above, a steady state is roughly thirty to forty reviews a day, which is about three minutes. That is the number to design your habit around. If the app makes you do more than that, it is asking too much and you will stop.

Where the words come from

Lists work for exams. For everything else, the words worth learning are the ones you actually meet while reading, because you already have a reason to care and a sentence to anchor them. Capturing words as you read is the other half of this; spaced repetition is what makes the capture worth doing.