Desirable difficulty for effective learning

When we are presented with new information, we try and connect it to information we already hold. This is automatic. Sometimes the information fits in easily; other times the fit is more difficult — perhaps because some of our old information is wrong, or perhaps because we lack some of the knowledge we need to fit them together.


D’Mello, S., Lehman B., Pekrun R., & Graesser A. (Submitted). Confusion can be beneficial for learning. Learning and Instruction.

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The most effective learning balances same and different context

I recently reported on a finding that memories are stronger when the pattern of brain activity is more closely matched on each repetition, a finding that might appear to challenge the long-standing belief that it’s better to learn in different contexts. Because these two theories are very important for effective learning and remembering, I want to talk more about this question of encoding variability, and how both theories can be true.

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The most effective way of spacing your learning

We don’t deliberately practice our memories of events — not as a rule, anyway. But we don’t need to — because just living our life is sufficient to bring about the practice. We remember happy, or unpleasant, events to ourselves, and we recount our memories to other people. Some will become familiar stories that we re-tell again and again. But facts, the sort of information we learn in formal settings such as school and university, these are not something we tend to repeatedly recount to ourselves or others — not for pleasure anyway! (Unless you’re a teacher, and that’s part of the reason teaching is such a good way of learning!)

So, this is one of the big issues in learning: how to get the repetition we need to fix something in our brain. Simple repetition — the sort of drill we deplore in pre-modern schools — is not a great answer. Not simply because it’s boring, but because boring tasks are not particularly effective means of getting the brain to do things. Our brains respond much better to the surprising, the novel, the emotional, the interesting.

Teachers today are of course aware of this, and do try (or I hope they do!) to provide as much variety, and interest, as they can. But there is another aspect to repetition that is less widely understood, and that is the spacing between repetitions. Now the basic principle has been known for some time: spaced repetition is better than massed practice. But research has been somewhat lacking as to what constitutes the optimal spacing for learning. Studies have tended to use quite short intervals. But now a new study has finally given us something to work with.

For a start, the study was much bigger than the usual such study — over 1350 people took part — increasing the faith we can have in the findings. And, crucially, the interval between the initial learning session and the second review session ranged from several minutes to 3.5 months (specifically, 3 minutes; one day; 2 days; 4 days; 7 days; 11 days; 14 days; 21 days; 35 days; 70 days; 105 days). The time until test also covered more ground — up to nearly a year (more specifically: 7 days; 35 days; 70 days; 350 days). The initial learning session involved the participants learning 32 obscure facts to a criterion level of one perfect recall for each fact. The review session involved the participants being tested twice on each fact. They were then shown the correct answer. Testing included both a recall test and a recognition (multi-choice) test. The participants, by the way, ranged in age from 18 to 72 years, with an average of 34 (the study was done using the internet; so nice to get away from the usual undergraduate fodder).

So there we are, a very systematic study, made possible by having such a large pool of participants (the benefits of the internet!). What was found? Well, first of all, the benefits of spacing review were quite significant, much larger than had been seen in earlier research when shorter intervals had been used. Given a fixed amount of study time, the optimal gap, compared to no gap (i.e. 3 minutes), improved recall by 64% and recognition by 26%.

Secondly, at any given test delay, longer intervals between initial study session and review session first improved test performance, then gradually reduced it. In other words, there was an optimal interval between study and review. This optimal gap increased as test delay increased — that is, the longer you want to remember the information, the more you should spread the gap between study and review (this simplifies the situation of course — if you’re serious about study, you’re going to review it more than once!). So, for those remembering for a week, the optimal gap was one day; for remembering for a month, it was 11 days; for 2 months (70 days) it was 3 weeks, and similarly for remembering for a year. Extrapolating, it seems likely that if you’re wanting to remember information for several years, you should review it over several months.

Note that the general rule is absolute rather than relative: when measured as a proportion of test delay, the optimal gap declined from about 20 to 40% of a 1-week test delay to about 5 to 10% of a 1-year test delay. In other words, although the optimal gap between study and review increases as the length of time you want to remember for increases, the ratio of gap to that length of time will decrease. Which seems very commonsensical.

As the researchers point out (and as has been said before), “the interaction of gap and test delay implies that many educational practices are highly inefficient”, concentrating topics tightly into short periods of time. This practice is likely to give misleadingly high levels of immediate mastery (as shown in tests given at the end of this time) — performance which is unlikely to be sustained over longer periods of time.

It’s also worth noting that the costs of using a gap that is longer than the optimal gap are decidedly less than the costs of using a shorter gap — in other words, better to space your learning longer than too short.

This article first appeared in the Memory Key Newsletter for December 2008

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Flashcards are cards with a word (or phrase) on one side and its translation on the other. You can buy ready-made flashcards, and these can certainly be helpful, particularly if you're inexperienced at learning another language. However, it is more effective if you make them yourself. Not only will the cards be customized to your own use, but the activity of selecting words and writing them down help you learn them.

A standard way of using flashcards is simply to go through a set number each day, separating out those you have trouble with, so you can review them more often. Keep these ones handy so that you can go through them at odd moments during the day when you're waiting for something.

Use the flashcards as a handy way to group words in different ways. Deal out the cards and move them around, looking for connections.

If you have word-family flashcards (recommended) - e.g., cards with various related forms of a word - you can make different sentences with your cards. You could also play cards with them, if you have others to play with. You could play a version of rummy, for example, where the sets are infinitive, present tense, future tense, past perfect. Use your imagination!

A bingo game with flashcards is another fun way to practice. Construct bingo cards (large cards divided into a certain number of spaces the same size as your flashcards) with the native language words on it. While this is better played with others, you can at a pinch play with yourself, simply picking out a flashcard from the pile and seeing how quickly you can match it with its counterpart.

Learning words in isolation will not help you much in dealing with words in context. You do need to practice reading/writing/speaking/listening sentences. But flashcards are a useful means of memorizing vocabulary.

Flashcard software

VTrain (Vocabulary Trainer): is flashcard software apparently used in the language labs of 40 Universities and hundreds of high schools; it's free for educational establishments. It's shareware.

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Spacing your learning

  • Spacing your learning / practice is more effective than doing it in long concentrated blocks
  • People commonly over-estimate how much they've learned, after a concentrated block
  • Memorization of items during a study session is most effectively done by recalling items at increasing intervals

Distributed practice more effective than massed practice

It has long been known that spacing practice (reviewing learning or practicing a skill at spaced intervals) is far more effective than massed practice (in one heavy session). An interesting example of this comes from a study that aimed to find the best way of teaching postmen to type (this was at the request of the British Post Office). The researchers put postmen on one of four schedules:

  • an intensive schedule of two two-hour daily sessions
  • one of two intermediate schedules involving two hours a day, either as one two-hour session, or two one-hour sessions
  • a more gradual schedule of one hour a day

The researchers found quite dramatic differences, with the one-hour-a-day group learning as much in 55 hours as the four-hour-a-day group in 80. Moreover, the gradual group showed greater retention of their skills when tested several months later.

Research has also demonstrated that people commonly over-estimate the value of massed practice, and tend not to give due recognition to the value of spacing practice. This particular study confirmed this, by finding that, notwithstanding their superior performance, the gradual group were the least happy with the program - for though they learned much more quickly in terms of hours, it took them many more days (80 hours at four hours a day is 20 days, but 55 hours at one hour a day is 55 days).

Micro-distribution practice

What about practice over much shorter intervals? Say you are learning vocabulary in a foreign language - is it better to repeat a word twice in rapid succession, or to space out the repetitions?

On the basis of the distribution principle, the answer is clear. Go through your list once, then repeat it. That way, every item will be maximally distant from its own repetition. But the distribution principle isn't the only memory principle at work here. The other principle is that of generation - that if you produce the word for yourself, this will strengthen the connection better than having the word given to you. And your likelihood of being able to successfully recall the word is greater if you test it earlier.

So you have two opposing principles at work here: one says maximise the time between repetitions, the other says minimise it. Which wins? Well, neither. They're both at work, so you need to take both into account, like this:

  • the first time, test a new word after only a brief interval (your own experience is best here, to tell you what length of interval is best for you)
  • on successive recalls, gradually increase the interval (your aim is to find the maximum interval at which you can reliably recall the word)
  • if you fail to recall the word, shorten the interval; if you succeed, lengthen it

Distributed practice in skill learning

The distribution principle also applies to skill learning, although people are probably even more reluctant to apply it. Practicing a skill in a concentrated block seems to give better performance, and indeed it does - at the time. The problem is, it doesn't lead to better long-term learning.

Part of the problem is that it makes you over-estimate how well you have learned the skill. But most of that learning will fade quickly. To learn the skill properly (i.e., for over the long term), you are best, not simply to distribute your practice, but also practice the skill in the context of a variety of different tasks. For example, if you were learning to type, you could hammer away at one combination of keys (say, asdf) thirty times, then you could move on to another sequence (jkl;) and repeat that thirty times, and so on. But it would be better if you mixed the sequences up.

It is thought that practicing in this way works better because it requires you to repeatedly retrieve the motor program corresponding to each task. It also requires you to differentiate the skills in terms of their similarities and differences, which may be assumed to result in a better mental conceptualization of those skills.


  1. Baddeley, Alan. Your memory: A user’s guide. (2nd ed.) London: Penguin Books, 1994.
  2. Simon, D.A. & Bjork, R.A. 2001. Metacognition in Motor Learning. Journal of Experimental Psychology: Learning, Memory and Cognition, 27 (4).

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