Spaced Repetition Theory

Cecil Alec Mace, 1932

The Psychology of Study

Spitzer HF. Studies in retention. Journal of Educational Psychology. 1939;30:641-657.

Melton, AW. The situation with respect to the spacing of repetitions and memory. Journal of Verbal Learning and Verbal Behavior. 1970;9: 596–606.

Landauer, TK & Bjork, RA. Optimum rehearsal patterns and name learning. In M. Gruneberg, P. E. Morris, & R. N. Sykes (Eds.), Practical aspects of memory. 1978; 625–632. London: Academic Press.

“Dr. Brown, there’s a respiratory arrest coming into room 214. Let’s go. Have you ever intubated before?”

“It’s only my 2nd shift, Dr. Hafner, but I did intubate once as a medical student.”

“No problem, I’m here to back you up. What medications do you want to use?”

As his attending asks the question, Dr. Brown vaguely remembers a resident calling for medications last year and guesses succinylcholine and etomidate.

“Great. Do you remember the contraindications for succinylcholine?” Dr. Hafner asks.

Dr. Brown sighs.  “I used to know the contraindications for succinylcholine but that was first year of medical school, Dr. Hafner. I don’t remember what they are anymore.”

As they prepare for the intubation, Dr. Hafner reminds Dr. Brown of what he memorized years ago.  As Dr. Brown sits down with Dr. Hafner after his successful intubation, they discuss the difference between cramming for tests and retaining learned information.  Dr. Hafner points out to Dr. Brown that in the past he memorized a lot of information necessary to pass tests but not directly applicable to his chosen profession, such as histology slides or the steps of the Krebs Cycle.  What he learns moving forward in his residency, however, will apply to his patients every day and the rest of his emergency medicine career. Retention of what he learns will be an important part of his studies and it will be essential for him to successfully remember what he needs to know to help his patients.

Dr. Hafner recommends Dr. Brown look up spaced repetition and incorporate it into his study plan. They set a time later in the week to follow-up.

Imagine you have just enrolled in a new course at the local university.  The material is going to be quite difficult for you and you begin to wonder: how should I go about learning all of this new information?  If you talk to 10 different people, you are likely to get 10 different answers about which approach is best.  Some prefer to do their studying all at once, others will study over time. Still others will chunk all of their similar material together and some may prefer to study a little of everything each time they sit down.  Each will tell you theirs is undoubtedly the ideal approach.

The theory of spaced repetition suggests there is, in fact, an optimal approach to studying to maximize retention: spreading out the material over time and periodically refreshing old content.  This may seem counterintuitive at first: shouldn’t we do as much of our learning as close as possible to when we are going to be assessed on it?  Shouldn’t we master one topic before moving on to another?  Empiric data suggests that neither of these are the case if the goal is to maximize long-term retention of knowledge.  Read on to discover what the research has taught us so far about the science of learning and retention.

The Forgetting Curve

Forgetting occurs exponentially.1This simple but profound insight, from Ebbenghaus’ experiments on himself in 1885, suggests that we are programmed to forget, and retention of information needs to be an active process.  If we are trying to retain knowledge, there are ideal moments to study the material again: not right away after just learning it, but not years later either. The best time appears to be when the knowledge has just started to fall out of your conscious awareness.

Ebbenghaus created the “forgetting curve” (Figure 1) to illustrate this exponential loss of knowledge.  The lack of frequent reviews or spaced practice of the material to help solidify and reinforce the information results in an exponential fall and inability to recall this information at a later date.  However the spacing of reviews is key, as very little decay of the material occurs hours to days after accruing new information.

Spaced Repetition vs. Massed Repetition

Most people would agree that a single exposure to information is not sufficient to produce good long term retention, helping to support that phrase “practice makes perfect”. However, there are multiple studying techniques that have been used to try to optimize the learners’ effectiveness and efficiency in learning.

Spaced repetition is the theory that intermittently returning to material after time away leads to more durable retention of the knowledge and incorporation into long term memory.1,2 Also known as distributed practice or the spacing effect, these short learning bursts help to reinforce concepts and improve long term recall of the subject matter.  Spaced repetition is considered the opposite of “massed repetition” (sometimes termed “cramming”) where a learner attempts to take in all of the material at once, never to return to it again.  Cramming is a framework for learning beginning in elementary school and continuing through adulthood.3 Often topics are learned and tested separately prior to moving onto the next topic, making the massed repetition approach effective enough to receive a high grade in school.

While massed repetition may be viewed as more favorable because it delivers the end product now (e.g. the night before a test), long term retention is much lower.1,3,4 The evidence for the effectiveness of spaced repetitions is strong.  A recent comprehensive review of the utility of various studying techniques and strategies has awarded distributed practice or spaced repetition as one of the highest rating based on available research evidence.4

Kang2 contrasts spaced versus massed practice as shown in the figure below:

Logistics of Spaced Repetition

• Time interval:

Studies on spaced repetition do not control for the time of day (night versus first thing in the morning).1 However despite this, the studies yield similar results suggesting that the time variance has minimal impact on memory retention in spaced repetition.  It is should be noted, though, that memory consolidation into long-term memory does occur during sleep, suggesting that reviewing at bedtime may be optimal.

• Spacing of Repetition- Static vs. Expanding intervals:

The ideal length of the time between learning episodes has not been determined definitively, although several studies suggest fixed spacing is superior to expanding.1,4 (Gwern, Dunlosky) Various methods have been tried, such as using incorrect responses or learner reported confidence to determine when the material should appear next.  Ultimately, the exact length of time may not be as important as adherence to the general principle of multiple exposures to the material divided across a length of time2 (Khang).   However, allowing an extraordinary amount of time between intervals can result in the learner have to relearn the material as if it was the first time they had studied it.

The amount of time the learner desires to retain the information also affects the spacing. For example, in order to remember something for 1 week, the learning episodes should be spaced 12 to 24 hours apart; to remember something for 1 year, a lag of 2 months is appropriate; to remember something for 5 years, the learning episodes should be spaced 6 to 12 months apart.2,4

Thio5 illustrated this principle below:

The Testing Effect

Testing, as an active component to learning, is much more effective than just studying facts.6 Spaced repetition is also closely related to the idea of retrieval practice or the testing effect, which states that forcing oneself to actively recall information (i.e. testing) is a more effective learning technique than more passive approaches such as simply reading or hearing the information again.  Since many applications of spaced repetition use self-testing with flash cards or questions, learners can reap the benefits of both effects.   There is debate over the ideal time to test the material.  There are benefits to testing the moment new material is learned, although the tests can also be separated by time, thus creating a spacing effect or spaced repetition.  Multiple studies found that when the final test/measurement is conducted days or years later, spacing tests is superior to massed testing.1-4 An important component of the testing effect is the initial feedback of whether the answer was correct.  Feedback enhances the learning, although even without feedback, testing was found to be beneficial.6 The best results were found when delayed feedback was given, which reflects the spaced component, further improving the learning and retention of material.

There are a variety of ways in which spaced repetition is relevant to the modern medical educator.  Any situation which calls for learners to be responsible for a large volume of material may benefit from application of spaced repetition.

• Weekly didactic lectures1,4,6

Often didactic content is organized by subject matter blocks (e.g. cardiology, infectious disease), which places content into one concentrated period, with months or even years before the content cycle repeats.  Spaced repetition suggests an approach which consistently mixes content areas with frequent, intermittent reminders of key topics, creating opportunities for learners to recall recently learned information.

• Question banks1,2

Many residencies utilize question banks (e.g. Rosh Review or HippoEM) to reinforce recently-learned content.  This is a great example of retrieval practice in action; as learners work to recall the information, they cement it further in their memory.  Spaced repetition also suggests that rather than quiz learners on recently-learned material, educators should use questions to refresh material learned longer ago that may be falling out of memory and vary the subject matter of the questions they are using.

• Flash Cards1,4

Flipping through cards can be an excellent way to solidify critical associations.  Seeing the same cards that you’ve already mastered however, can waste valuable study time that would be better used on refreshing knowledge that is not as well established in your memory.  Flashcard software (e.g. Anki and SuperMemo) uses the number and frequency of your correct responses to determine the optimal time for you to see a card again.  Get one wrong, and you’ll see the same card again soon to cement your memory.  Answer correctly several times in a row and the card will disappear from the regular rotation for a longer interval, allowing you to study other cards instead.

Future Applications

We’ve all sat through a lecture or read a book chapter thinking to ourselves: “I know this stuff already.”  In an ideal world, we’d never have to experience this because all of our learning would take into account our recent experiences, giving us only new knowledge or refreshing us on what we’re about to forget.  A future residency program might expand on the promise of Anki’s flashcards, generalizing the principle to encompass all of a resident’s learning opportunities to create just-in-time learning, personalized for what each resident’s needs.  For example, a third year resident who has just seen 5 cases of chest pain and read 2 articles on abdominal pain may be required to do lesson on the evaluation of eye problems.  Another third year resident who has seen several neurologic complaints and has been reading about pharmacology might need a lesson on the evaluation of chest pain instead.

As noted by many researchers, the spacing effect is “a case study in the failure to apply the results of psychological research.”7 Many hurdles exist to the application of spaced repetition practice into classrooms including many teachers’ default to other familiar techniques.  Also conventional instruction favors massed practice, and homework assignments typically do not rely on long term memory but rather knowledge immediately acquired in the preceding class.2

Applications in the clinical setting1

• Alzheimer patients:  Spacing used on the scale of seconds and minutes showed modest success in teaching object location or daily to-do tasks.
• Traumatic Brain Injury patients:  a pilot study showed improvement in learning and memory for functional tasks
• Multiple sclerosis:  spacing effect was shown to help with functional applications
• Procedural tasks: there is some conflict on how much spaced repetition applies to motor skills but although many found that spaced repetition can help with procedural tasks such as suturing, typing, even certain sports such as archery, javelin throwing

How to learn Effectively in Medical School: Test Yourself, Learn Actively, and Repeat in Intervals –  Marc Augustin

Focus for medical students with a discussion on how to improve learning in medical school by using spaced repetition and the testing effect.  

Spaced Repetition: The Most Effective Way to Learn- Josette Akresh-Gonzales

Brief explanation of massed vs. spaced repetition with a link to video discussion with Dr.  Robert Bjork, UCLA chair of psychology, an expert in human learning and memory.  

Spaced Repetition Promotes Efficient and Effective Learning: Policy Implications for Instruction – Sean H. K. Kang

In-depth discussion on spaced repetition and the logistics of this study technique in enhancing memory retention.  

Improving Students’ Learning with Effective Learning Techniques: Promising Direction from Cognitive and Educational Psychology- John Dunlosky

Comprehensive review of various learning strategies and how to implement the various techniques.  Section 9 discusses Distributed Practice (also termed spaced repetition) with a recent review of top articles and graphs to discuss forgetting curve and how spaced repetition combats against this.  

Spaced repetition has been found to be less helpful in complex subject matters such as philosophy, literature, creative writing or math computation.  Specific concerns in the testing aspect of spaced repetition include1:

• Interference to Recall:  the ability to remember tested items drives out our ability to remember similar untested items, leading to preferential memory of tested items.
• Multiple choice testing:  inadvertently leads to “negative suggestion effects” wherein a previously seen falsehood in an item on a test makes the learner more likely to believe it.  The way to guard against this is quick feedback about the right answer or avoidance of multiple choice questions.

Other pitfalls of spaced repetition include:

• Formulating poor questions and answers
• Assuming it will help you learn, rather than preserving what you have already learned (ie: it’s difficult to learn from cards, but if you already know the information, using flashcards to refresh the information can identify weak points and strengthen knowledge).
• Material overload:  new users tend to add too much information or include trivial information, leading to lower motivation to do repetitive spaced reviews.

At his follow-up meeting with Dr. Hafner, Dr. Brown shares what he learned about the importance of spaced repetition and they work together to incorporate it into his study plan. Dr. Brown downloads the flashcard app Dr. Hafner recommends. The app tracks his responses and spaces the frequency of which the flashcards recur for optimal spaced repetition. Dr. Hafner also points out that spaced repetition is one of the reasons their residency’s weekly didactic session curriculum was changed to avoid subject blocking, similar to his medical school curriculum.

As Dr. Brown builds his flashcard bank throughout his residency and uses it for frequent review, he soon finds improvement in his retention compared to medical school. He follows up again with Dr. Hafner and proposes a plan for sharing a flashcard bank with the residents built from their weekly didactic sessions, further incorporating spaced repetition into the residency curriculum.

Dr. Brown prepares to graduate from residency and start his career in emergency medicine, and he reflects on the difference three years has made in his fund of knowledge. He knows that while he still has a lot to learn, he has a  solid base of retained knowledge that will serve as a foundation for his future career.