Drake Thomas

Introduction / TLDR

I use spaced repetition systems (specifically Anki) for a variety of purposes, and they form a fairly core part of my intellectual environment. Unlike every other such person on the internet, though, I don't think you should obviously use them - if you're like me you probably should, but for many people it might be a bad call! The rest of this page dives into what I see as the pros and cons, and some less standard use cases for spaced repetition that I've found helpful.

What is this thing?

Spaced repetition software, or SRS, refers to any of a variety of applications for creating flashcard-adjacent content and reviewing these cards over longer and longer intervals as you get to know them better. My impression of the scientific literature is that this is pretty reliably shown to be the best way to establish long-term recall of a body of knowledge, and my ancedotal experience is that it works quite well.

To a first approximation, if you want to learn things that can fit in flashcard form, spaced repetition is the best way to do it, and (as I'll argue) rather a lot of things can work in flashcard form.

For more information about spaced repetition, gwern's page provides a great overview.

Should I use it?

Spaced repetition is generally useful to the extent you want to memorize a lot of things. I delight in adding new facts and skills and general knowledge into my brain, and so I have thousands of flashcards in my Anki deck (and hundreds more on my list of "things I should add to my Anki deck sometime soon").

If you already think you want to memorize a lot of things, you may need no further convincing. If you already know you hate memorizing things, feel free to close this tab. But I claim that many people haven't really considered the space of things they might be able to usefully add to their memory if they had good tools for doing so, so I'd like to list some different flavors of memorization one might try deploying SRS for. See which of them strike your fancy.

What could I use this for?

Listed roughly from most to least orthodox:

Normal stuff: locations of countries in the world, elements of the periodic table, digits of pi, state capitals, vocabulary for a foreign language, etc. The central examples of "stuff people delibrately memorize". Many of these things have real value, many of them just appeal to the sort of person who likes checking off this flavor of accomplishment.
Retaining things you've studied: I used to have flashcards cached as "that thing students more diligent than me do instead of cramming when they want to do well on the test". When you want more than to do well on the test, when you want to actually have the contents of that chemistry class or linguistics seminar or whatever to stick around in your brain for the rest of your life, you can distill its contents into flashcards for your SRS deck and never forget it again. (Spaced repetition works well for the test-prep thing too if you want to delete all the cards after, but it's not the way I orient to my deck.)
Random facts: If you train the reflex to go from "huh, that's a neat fact" to "let's put this in Anki", let that feedback loop run for a few years, and follow your curiosity, you will become noticeably better-informed about the sorts of things you find interesting, and people may start thinking of you as an unusually knowledgeable person. What if you could recall everything you've ever learned that made you go "oh, that's neat"?
Poetry: I really like having interesting poems in my head for recitation at will; sometimes it comes up socially, sometimes it helps with seeming cultured, a lot of the time I just find it nice to be able to recite a poem I like to myself on a walk. Poems I particularly enjoy include Hymn of Breaking Strain, Song of Eärendil, and The Cremation of Sam McGee.
Faces and social information: If you have access to a list of names and faces (or profile photos) before an event, you can try to boost your ability to recall who's who by pre-memorizing the correspondence. I've done this once, and I think usually the costs outweigh the benefits, but it might be useful if you care unusually much about the event or have particularly poor memory for faces.

You may also want to remember things like birthdays, where a friend lives, cues to social habits you'd like to instill (e.g. "what should you do when you come across a beautiful website?" / "send it to [friend] for their collection of CSS inspiration ideas"), etc.
Things you enjoy talking about: I love puzzles and posing them to other people, so I’ve got lots of flashcards that prompt me with “Puzzle about [topic]” whose answer is some interesting puzzle I’ve collected over the years. (If I think I’m liable to forget the answer myself, there might be a second card posing me the puzzle to jog my memory of the solution.) To take a more absurd example, I reference some specific things (useful papers, xkcd comics, etc) often enough that I memorize their title or URL to save myself a google search. Any time you observe yourself interrupting a conversation to do "hang on, what was the thing again?" and Google something for the third time in a row, put the fact in an SRS deck so it doesn't happen again.
Fermi estimation: I think a big piece of being good at estimating things on the fly is having a bunch of relevant pieces of your world-model accessible as numerical values. Do you know roughly how large the Earth is, the population of the US and its GDP, the density of a typical rock, the concentration of CO2 in the atmosphere, your odds of death from a motor vehicle accident per year? You can just have the 100 most important such facts accessible to you all the time! I find this to be a pretty big multiplier on my ability to estimate things, sanity check my observations about the world, and put things in context.
Reflections: This is a category I make frequent use of: it's for things that I'd like to stare at every now and then, but which I don't actually want to remember the entirety of. This is a sort of fuzzy category encompassing many things where I care more about getting the general gist of things, or where I care about rereading the content but not remembering it.

Things I might put in here:
- Graphs: When I see interesting charts or diagrams in papers / blog posts / my own coding, I'll often put them in a card whose front is "Reflection: graph of [thing]" and whose back is the corresponding image with a link to a source.
- Quotes: Favorite speeches, epigrams, poems, paragraphs from blog posts, etc. Sometimes I care more about reabsorbing the vibe of a particular text more than regurgitating its contents verbatim, and this can be a useful way to do so. If you're into things of this flavor, also consider incremental reading, though I can't speak to its efficacy myself.
- Notes: I'd like to do this more often, but one high-effort way to include more cards of this type is to distill complex things you've learned into their basic ideas, then add a reflection card to boost your memory about what was up with a particular essay / paper / theorem / historical event, even if you don't need to know the precise wording / results / statement / dates in perfect fidelity.
- Art: Just pretty pictures that I enjoy looking at. Sort of an abuse of the spaced repetition idea, but in the end you can use these systems for anything you'd like to see on an occasional basis.
- Memories: I haven't played with this one much yet, but if you wanted to use SRS to enhance episodic as well as semantic memory, you could put summaries of events in your life you'd like to recall later into a reflection-type card and see them on a regular basis. (If you've done something like this, I'd love to know how it went!)
One particularly useful feature of the reflection category is that, because my Anki cards are searchable from my phone and computer, I now have an easily-indexed library of all the random graphs and quotes that I've ever thought were interesting, so it doubles as a reference for a lot of the things I want to point people at (and in fact if I notice myself reaching for a particular chart or quote to show someone, I might consider whether to add it for that purpose).

What are some drawbacks?

The biggest and most straightforward is just the time cost. It's not huge for standard use cases - five minutes a day should be enough to keep even a deck with several thousand cards in check, once you've learned them well enough that a given card only shows up rarely - but it's not nothing.

I also don't want to discount the more subtle drain on cognitive resources that comes with having yet another thing to track. In theory, you have room for 288 things that consume five minutes each day, but I don't think you can actually keep tabs on more than a dozen or two before the overhead on the meta level consumes all of your slack.

You might also just find it unpleasant! This is especially common at the beginning, when someone's cards are often

unfamiliar (because they haven't learned most of them well yet)
homogenous (because they've only added one or two categories)
poorly-designed (because they've spent no time getting good at making flashcards yet)

and getting over that hurdle can require willpower. (And it's not guaranteed you will get over it - some people might just always find it aversive.) I talk about some ways to avoid this failure mode further down, but at the end of the day some people like things more than others.

It's easy to spend a week or two putting off reviewing one's cards, and then having an ugh field around touching the hundred-card pile that's built up in your absence. The best antidote is to cultivate a sense of reward from making even a little progress each day; the second-best antidote is to sit down one afternoon and chain yourself to your SRS application until they're all dealt with. (I have this issue a lot; my days are around equally split between being on top of my cards, ignoring them, and making steady progress from the second state to the first.)

For my values and preferences, the benefits outweigh the costs, and I suspect that they do for a lot of others too! But the costs aren't zero. I weakly recommend giving things a try for a few weeks to a month with a relatively small deck (30-50 cards?) and seeing what you think.

I'm sold, what are some tips for how to use this well?

I'll plug gwern's page yet again for a great go-to reference on a lot of this stuff, but some particular tips I wish I'd had earlier:

Anki is probably the way to go, if you're unsure which system to use. I started out with Mnemosyne, which I now regret; while its tagging and browsing features are a more to my liking, the lack of an iOS mobile app and more clunky bits of the UI were fairly costly. I ended up switching over all of my cards in early 2022, which took a fair bit of energy to get sorted out. (I have heard some people extol the virtues of SuperMemo but haven't tried it myself. Empirically its advocates are more fanatical about their evangelism - I won't speculate about the causality there.)
I recommend having all your cards in a single deck, rather than segregating by topic; I find it more enjoyable if my reviews span a variety of subject matters, and you can use the tagging system to bucket your cards as finely as you want anyway.
Related to the above, I enjoy reviewing my cards more if there are a variety of topics I get to engage with: if I have one hundred facts about Git to learn and nothing else, I will feel pretty meh about it, but if one moment I'm locating Afghanistan on a map and the next I'm answering a question about tidal forces and the next I'm reciting poetry and the next I'm thinking about constellations, the experience is more enjoyable.
Many people before me have opined on the proper methodology for making good flashcards: see [TODO]. The core principles are to make the mental motion from card to answer as atomic as possible, and to prompt yourself with the sort of context you actually want to be able to remember the thing in.
If you start off with 10 cards, you may feel like the setup costs of engaging with an app that only shows you 1-2 of them per day aren't worth it; if you start with 200, you might add them all in one go and then face a mountain of reviews and get dejected. A few dozen is a reasonable place to start. (Starting with 200 probably means you downloaded them from some deck online, too, which can lead to less engagement because the prompts weren't written in a style that works best for you.)
Cloze deletion cards are pretty great for a lot of purposes; they're especially nice for turning relatively little card-creation work into a lot of relevant flashcards that are bite-sized in the right way (instead of a bunch of duplicated work rewriting custom cards, or a single massive card that requires loading ten things in your head).

Image occlusion cards (see the Anki extension; I found the video tutorials useful when I got confused) are also really great when you have diagrams or maps or something, but this is definitely more of an advanced feature.