Why we can’t remember what we learn and what to do about it

While our ability to store new information isn’t limited, our ability to access that information is limited by a variety of factors. However, a relatively easy technique has been proven to boost retention of learned materials: spaced repetition. Spaced repetition can be applied by any instructor to improve student learning outcomes in the long term^[1].

A history of forgetting

In the 1880s, German psychologist Hermann Ebbinghaus became fascinated with the science of forgetting. He theorized that forgetting is related to the passage of time. Setting out to prove this theory, he threw himself into a multi-year systematized study that tracked memory decay over time.

To understand how decay worked, Ebbinghaus created 2,300 nonsense sounds (rur, mek, hek) and memorized these, testing himself and varying the duration and number of practice sessions. By keeping track of how many times he had to rehearse each list of words and for how long, over many years, Ebbinghaus created the forgetting curve, a graph of memory loss over time^[2]. The forgetting curve shows us that we forget most of what we learn in the first day, and our memory decays over time unless we practice what we learn at intervals. Ebbinghaus kept track of how long it took him to commit his nonsense words to memory, creating an equation, later plotted as a graph^[3].

The Ebbinghaus forgetting curve of nonsense words

Ebbinghaus was the first to study memory by applying quantitative methods. While his study isn’t perfect, his careful measurements kicked off the study of memory using a clear methodology and did a lot to advance the field of psychology as a quantitative science. In the intervening years, other psychologists have assessed and modified his theory and worked to understand the relationship between forgetting and learning^[4].

Spaced repetition is a way of overcoming the forgetting curve

What we know. We know that multiple studies of spaced repetition (or spacing out study sessions) show that spaced repetition is a powerful way to remember something in the long term. Studies spanning grade school students to college students, using a variety of techniques – from studying word pairs to studying sophisticated biology concepts – show that students who used the spaced repetition technique outperformed students who crammed (or studied the material only once)^[5]. Even in tests of transfer, in which students made inferences and applied what they learned in one setting to another, the students who took part in the spaced repetition conditions did better than those who crammed.

Most students cram for tests. It seems like the intuitive thing to do and it calls for far less planning. And cramming works, but only in the short term. Students will remember what they learned for the test the next day, but when tested a month or a year later, students who crammed will have a lot of trouble accessing the material. Since the goal of learning is to build on what you know and transfer that knowledge to new situations, students who do not use the spaced repetition technique, are placing themselves at a disadvantage.

What we don’t know. Optimal spaced repetition allows memory to decay a little bit before reactivating it through study^[6]. Rather counterintuitively, some forgetting has to happen for spaced repetition to be productive. When students cram, or even use too short intervals between study sessions, their memory is still active, and restudying doesn’t activate it. However, when the intervals between practice sessions are too long, the memory cannot be retrieved^[7].

While numerous studies show the benefits of spaced repetition, there is a lot we don’t know, including the optimal gap between spaced repetition sessions. Should they increase with every interval (going from one day between sessions, to one week, to one month)? Or should they remain the same? While the general consensus is that spaced repetition schedules should involve wide spacing, we don’t know the optimal spacing for every circumstance; there is no strong evidence that one schedule of spaced repetition is better than another^[8]. While the timing has yet to be optimized, spaced repetition remains one of the most powerful, and long-term learning techniques that we can easily deploy. The key is recognizing the impact of the technique and putting it into practice.

What you should do. Instructors can address the problem of the forgetting curve and take advantage of spaced repetition using a process called interleaving. Interleaving introduces spacing of topics so that topics are not just covered consecutively but make an appearance in other lessons, assignments, or tests. When you interleave your topics, students have to continually adapt to each new topic and recall what they learned in previous classes. This prompts students to both actively retrieve previously learned information and surface connections or relationships between related ideas^[9].

You can include spaced repetition in your course by interleaving what you teach:

• Time your lessons so that you cover some of one topic, move on to another, and then get back to the original material.
• Design your exams and quizzes so that they cover more than one topic or set of related materials. Let your students know that you are doing this so that they know that they should study not just one topic but multiple topics, reactivating their memory of previous lessons.
• Insert previously covered material into current lessons. This can be as simple as cold calling on students and asking them to make the connection: how does this concept connect to a previous concept?
• Design your assignments with curricular connections in mind. Ask students to connect several concepts studied and explicitly refer to earlier concepts in their assignments.

Talk to your students about the benefits of spaced repetition. Show them the forgetting curve and tell them that for long-term retention, cramming is not a good idea. Spaced repetition will allow them to access what they learned so that they can use it when it counts: on tests and at work.

For a fun and interactive look at spaced repetition, check out Nicky Case’s How to Remember Everything Forever-ish