Desirable difficulties

A desirable difficulty is something that slows performance during practice and improves it afterwards. Bjork introduced the term in 1994 and the list has stayed stable: space sessions out rather than massing them, interleave topics rather than blocking them, vary the conditions of practice, and use tests rather than presentations as study events. All of them make the practice session look worse. Items come back more slowly, more errors are made, and the learner feels less sure. All of them also produce better recall and better transfer weeks later. The word desirable carries the weight: a difficulty earns the label by that later result, not by being hard.

The Bjorks explain it with two quantities. Storage strength is how well a memory is woven into what you already know; retrieval strength is how accessible it is right now. Performance in the moment depends only on retrieval strength, and the conditions that raise retrieval strength fastest (rereading, blocked drills, immediate repetition) are not the ones that build storage strength. Worse, learners read retrieval strength as storage strength, so they choose the method that feels productive. Roediger and Karpicke's rereaders in 2006 were the most confident and remembered least. Spacing, interleaving and testing all work by letting retrieval strength drop a little, so the next successful retrieval has to do real work and storage strength grows.

Not every difficulty qualifies. Bjork's own caveat is that a difficulty is desirable only if the learner has the background to respond to it; past that point it is an obstacle, which is why a beginner handed interleaved problems in a subject they do not yet know gets nothing from them. The evidence is strongest in laboratory tasks with verbal material, measured over days and weeks, and thinner for years or for whole skills. And because learners rate these methods as worse, they are dropped the moment the choice is left to the learner, which is the main practical problem.