Leitner box vs algorithmic scheduling: which spaced repetition system should you use?

How the Leitner box works

Sebastian Leitner described a box system in his 1972 book on learning. You need three to five boxes and a stack of cards. All new cards start in box one. When you answer a card correctly it moves to the next box. When you answer incorrectly it goes back to box one.

Each box is reviewed on a different rhythm. A common setup is box one daily, box two every three days, box three weekly and box four fortnightly. Sources describe different intervals for the boxes, which tells you the exact numbers are a choice, not a rule. The system is simple enough to run on index cards or in a spreadsheet.

How algorithmic schedulers work

Software can store a due date for every card. SM-2, published by Piotr Woźniak around 1990 and used by SuperMemo and, in its original form, by many later apps, grades each answer and multiplies the previous gap by an “ease” factor. Cards you find easy get long gaps quickly. Cards you find hard get short ones. More recent schedulers use statistical models of memory to predict when you will forget; Anki, for example, now offers one called FSRS as an option.

Some tools use a binary choice, known or needs review, and a simplified version of the same idea. Fewer buttons means faster sessions and less deciding.

Strengths of each

Leitner is transparent. You can see exactly why a card is in front of you, you can run it without a device, and it makes a nice physical routine. It suits a small deck of high-value facts, for example 80 cards for a particular exam block.

Algorithmic scheduling scales. With a few hundred or a few thousand cards, tracking individual due dates by hand is impractical, and software adapts gaps to each card. It also gives you a daily count of what is due, which helps you plan your time. The risk is that a big due queue can pile up after a few missed days.

What the evidence says about the choice

There is research showing that spaced practice beats massed practice (Cepeda and colleagues, 2006, review), but strong head-to-head evidence that one scheduling method gives clinicians better exam results than another is, as far as we know, lacking. Claims that one algorithm is far superior should be treated carefully. The difference between spacing and not spacing is larger than the difference between sensible spacing methods.

How to choose

A few questions help.

• How many cards will you have? Under 100: Leitner is fine. Hundreds: use software.
• Where do you study? On a commute or a break, a phone app wins. At a desk, paper is fine.
• Will you review daily? If you tend to miss days, pick a tool that handles a backlog gracefully, such as one that limits new cards until the queue is down.
• Do you want to write your own cards, or use ready-made decks? Ready-made decks are easier to manage in an app.

Managing a backlog in either system

After a busy shift week you may come back to 200 due cards. Do not try to clear them all in one go. Review the oldest first for a fixed 20 minutes, stop adding new cards until the queue is under control, and accept that some cards will be marked wrong because the gap was long. That is normal and the system corrects it.

A worked Leitner week

Suppose you use four boxes with these rhythms: box one daily, box two every third day, box three weekly, box four every two weeks. On Monday you add ten new cards to box one and review them. You know six, so those move to box two. The four you missed stay in box one. On Tuesday you review box one only, with the four plus any new cards. You get three right and they move up to box two. On Wednesday it is box two day: the nine cards there. Seven are right and move to box three, and two go back to box one.

At the end of the first week your cards are spread across all the boxes. Cards you find easy are out of your way for weeks. Cards you find hard are in front of you every day. The system delivers spacing with nothing more than index cards, four envelopes and a calendar.

Hybrid approaches

Some people use both. A small paper deck of ten or twenty personal facts, kept in your pocket for waiting moments, and an app for the bulk of the material. The paper deck can carry the facts you most fear forgetting on the day, and it is easy to take through the last days. Others use an app to schedule but run a weekly paper check on the cards that keep failing.

There is no prize for purity. The aim is to retrieve the facts at the right times. If you start with a box system and your deck grows beyond what you can handle, moving to software is a normal step, and you lose nothing by it.

What can go wrong with a box system

Boxes are simple, and the simplicity creates three typical problems. First, intervals are fixed, so a card that is a little hard and a card that is very hard may sit in the same box. Second, a single wrong answer sends a card all the way back, which can feel harsh for a card you nearly knew. Third, as the deck grows you need to track which boxes are due each day, and it is easy to lose track after a missed week.

Some people soften the second problem by moving a missed card back one box instead of all the way to box one. That is a reasonable personal tweak. It is the sort of change that does not need evidence behind it, because you can see for yourself whether your recall is holding up.

Before you commit to software

If you do choose an app, read how it schedules, find out whether you can export your cards, and check what happens to your data if you stop using it. A tool that locks your cards into a closed format is a risk if you are building a deck over months.

Practical tips for running either system

Put a date on your calendar each week to look at your decks as a whole, whatever tool you use. Count how many cards are due, how many you answered wrongly last week and which topics they come from. If the same topic dominates, schedule a proper study session on it. Use the same card-writing rules in both systems, since a badly written card fails in a box as surely as in an app. And remember that the schedule serves the exam date. In the last week you may choose to override it and concentrate on the cards you still miss. The DHA exam guide and the free assessment show where recall fits in the wider plan.

Where Meddex fits

Meddex flashcards use a simple known or needs-review choice with an SM-2 style scheduler behind it, so you do not manage boxes or due dates yourself. If you prefer Leitner for a small personal deck, nothing stops you from using both. The post on Anki versus Meddex flashcards compares the options fairly.

Frequently asked questions