Brown

How big an interval, you ask? The simple answer: enough so that practice doesn’t become a mindless repetition. At a minimum, enough time so that a little forgetting has set in. A little forgetting between practice sessions can be a good thing, if it leads to more effort in practice, but you do not want so much forgetting that retrieval essentially involves relearning the material. The time periods between sessions of practice let memories consolidate. Sleep seems to play a large role in memory consolidation, so practice with at least a day in between sessions is good.

Something as simple as a deck of flashcards can provide an example of spacing. Between repetitions of any individual card, you work through many others. The German scientist Sebastian Leitner developed his own system for spaced practice of flashcards, known as the Leitner box. Think of it as a series of four file-card boxes. In the first are the study materials (be they musical scores, hockey moves, or Spanish vocabulary flashcards) that must be practiced frequently because you often make mistakes in them. In the second box are the cards you’re pretty good at, and that box gets practiced less often than the first, perhaps by a half. The cards in the third box are practiced less often than those in the second, and so on. If you miss a question, make mistakes in the music, flub the one-touch pass, you move it up a box so you will practice it more often. The underlying idea is simply that the better your mastery, the less frequent the practice, but if it’s important to retain, it will never disappear completely from your set of practice boxes.

Like interleaving, varied practice helps learners build a broad schema, an ability to assess changing conditions and adjust responses to fit. Arguably, interleaving and variation help learners reach beyond memorization to higher levels of conceptual learning and application, building more rounded, deep, and durable learning, what in motor skills shows up as underlying habit strength.

In interleaving, you don’t move from a complete practice set of one topic to go to another. You switch before each practice is complete. A friend of ours describes his own experience with this: “I go to a hockey class and we’re learning skating skills, puck handling, shooting, and I notice that I get frustrated because we do a little bit of skating and just when I think I’m getting it, we go to stick handling, and I go home frustrated, saying, ‘Why doesn’t this guy keep letting us do these things until we get it?’ ” This is actually the rare coach who understands that it’s more effective to distribute practice across these different skills than polish each one in turn. The athlete gets frustrated because the learning’s not proceeding quickly, but the next week he will be better at all aspects, the skating, the stick handling, and so on, than if he’d dedicated each session to polishing one skill.

Knowledge is more durable if it’s deeply entrenched, meaning that you have firmly and thoroughly comprehended a concept, it has practical importance or keen emotional weight in your life, and it is connected with other knowledge that you hold in memory. How readily you can recall knowledge from your internal archives is determined by context, by recent use, and by the number and vividness of cues that you have linked to the knowledge and can call on to help bring it forth

Psychologists have uncovered a curious inverse relationship between the ease of retrieval practice and the power of that practice to entrench learning: the easier knowledge or a skill is for you to retrieve, the less your retrieval practice will benefit your retention of it. Conversely, the more effort you have to expend to retrieve knowledge or skill, the more the practice of retrieval will entrench it.

First, that some difficulties that require more effort and slow down apparent gains—like spacing, interleaving, and mixing up practice—will feel less productive at the time but will more than compensate for that by making the learning stronger, precise, and enduring. Second, that our judgments of what learning strategies work best for us are often mistaken, colored by illusions of mastery.

This paradox is at the heart of the concept of desirable difficulties in learning: the more effort required to retrieve (or, in effect, relearn) something, the better you learn it. In other words, the more you’ve forgotten about a topic, the more effective relearning will be in shaping your permanent knowledge.

The more effort that is required to recall a memory or to execute a skill, provided that the effort succeeds, the more the act of recalling or executing benefits the learning.

the change from normal presentation introduces a difficulty—disruption of fluency—that makes the learner work harder to construct an interpretation that makes sense. The added effort increases comprehension and learning. (Of course, learning will not improve if the difficulty completely obscures the meaning or cannot be overcome.)13