michael_nielsen

To make this a little more concrete, at repetition number 5 (on the axis) a reader has reviewed all 112 questions in the essay 5 times.

And the y axis shows the total demonstrated retention. So, for instance, after 5 repetitions, most readers are up around 3,000 days of demonstrated retention. That means an average of about 3,000 / 112 ~ 27 days per question in the essay.

That's pretty good, I think. If you're anything like me, a few weeks after reading something you have only a hazy memory. By contrast, these people have, at very low cost (of which more below), got a month or so of demonstrated retention across more than 100 detailed questions

This is the big hoped-for advantage of spaced repetition, and what makes it counterintuitive and unusual: you get _exponential returns_ for increased effort.

It typically takes about 80 minutes of review time to get to 5 reviews. That's for an essay which takes about 3 hours to read - review for 80 extra minutes, and you can remember nearly all the details for about a month.

We think that with just a little extra marginal effort you'll remember those details - the entire body of knowledge - for more like a year. That's the benefit of exponential returns. But the prototype hasn't been out long enough to test that.