Spacing effect

The spacing effect is the finding that, in learning, the arrangement of practice, not just its amount, changes the outcome. Even with the same total time spent, cramming repetitions into one day may make performance look good in the moment, but spreading those repetitions over several days often leads to better retention later.

The first person to document this phenomenon was the German psychologist Hermann Ebbinghaus. In 1885, while conducting experiments on himself that involved memorizing meaningless groups of letters, he noted that spreading practice over several days was more efficient, even when the number of repetitions was the same. Over more than a century since then, results in the same direction have been observed repeatedly across different populations, ages, and tasks.

The familiar experience of remembering more from reviewing a little each day than from cramming for an exam reflects the same idea. The spacing effect can be seen as a theoretical framework for explaining this long-standing experience.

Rather than attributing it to a single cause, researchers use several interconnected explanations.

• Time for memories to settle — Immediately after learning, memories are still unstable and undergo a process called memory consolidation, in which they become more firmly established over time. This process is understood to be particularly active during sleep. Spacing creates time for it, whereas repeating everything at once leaves less time.
• The effort of recalling — After a gap, you have to retrieve what you learned earlier from memory. One explanation is that this effortful recall strengthens memory. When you repeat something immediately, it is still fresh in your mind, so there is no need to work at recalling it.
• A variety of cues — When you practice at different times, your location, mood, and physical condition also vary a little. Memories stored alongside multiple contexts have more cues available for retrieving them later.
• Concentration — When practice is crammed into one session, concentration fades and practice quality drops as the session goes on. Practical factors like these also play a role.

These explanations are also presented as a representative example of desirable difficulties: smooth performance during practice and lasting learning are not the same thing. Spaced practice feels a little harder while you are doing it, but that difficulty can actually help the learning last.

Research answers the question “How many days apart is best?” with “It depends on how long you want to remember it.”

Several studies, including a paper by Cepeda and colleagues (2006) that analyzed studies of verbal recall tasks, reported that the appropriate spacing increases as the intended retention period grows longer. In other words, shorter gaps are more advantageous if your test is a few days away, while longer gaps are more advantageous if you want to remember something for several months. This relationship gave rise to the rule of thumb “Increase the review intervals as you aim to remember for longer,” leading to spaced repetition methods that gradually lengthen the intervals between reviews.

Another point worth knowing is that this curve is fairly gradual. The fact that practice is spread out over time contributes more to the outcome than getting the exact number of hours or days right. So you do not need to put off starting while you work out the perfect schedule.

The spacing effect has been demonstrated most clearly in memorizing words and factual knowledge, and findings in the same direction have also been widely reported in motor skill learning. For motor tasks, however, the size of the effect varies considerably with the nature of the task and the length of the gaps. Gaps between practice sessions and rest periods between attempts within a session do not work in exactly the same way, either.

In the practical organization of training, this principle is discussed under distributed and massed practice. The recommendation to practice “briefly and often” rather than “for a long time all at once” in rehabilitation exercises or exercises done on your own follows the same reasoning. Early in learning a new movement, it is also useful to consider how to arrange the order of practice, as in blocked and random practice.

• Several short sessions rather than one long session — When learning a new exercise movement, try spreading short practice sessions over several days rather than doing a full hour in one day.
• Recall movements you learned before — At the start of practice, first try mentally recalling the movement you learned last time. The effort of recalling helps strengthen the memory.
• Do not stop just because it went well today — Performance on the day of practice and the skill that remains several days later can look different. Trying again a few days later and checking how much remains can tell you whether the movement has really become familiar. This check uses the same principles as retention and transfer tests.
• Do not let the gaps become too long — If the gap becomes long enough that you have almost forgotten the earlier material, the benefit decreases. Start with short review intervals, then gradually lengthen them as you become more familiar with it.
• Sleep is part of practice, too — Memories are understood to become established during sleep, so getting a good night's sleep on a day you have practiced can also help.