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Some study sessions feel great. You reread a chapter, solve ten problems of the same type in a row, and everything flows. Others feel clumsy: you mix topics, get stuck, and forget things you thought you knew. The uncomfortable finding from decades of memory research is that the clumsy session is often the one that sticks.
What are desirable difficulties?
The idea comes from UCLA psychologists Robert Bjork and Elizabeth Ligon Bjork. In their essay Making Things Hard on Yourself, But in a Good Way, they draw a line between performance, which is what you can see during practice, and learning, the lasting change that shows up later. Current performance, they write, "can be a highly unreliable index of whether learning has occurred."
Conditions that make performance improve quickly often fail to support long-term retention, while conditions that slow apparent progress often help it. They call these helpful obstacles desirable difficulties and list four main ones: varying the conditions of practice, interleaving topics instead of blocking them, spacing study sessions, and testing yourself instead of re-presenting the material.
The word "desirable" carries a warning. In the Bjorks' words, if the learner "does not have the background knowledge or skills to respond to them successfully, they become undesirable difficulties." Difficulty you can work through helps. Difficulty that leaves you completely lost does not.
The evidence behind each difficulty
Testing. Henry Roediger and Jeffrey Karpicke found in Psychological Science that students who practised recalling a passage remembered more after two days and a week than students who reread it, though rereading looked better on a test five minutes later.
Interleaving. In a study by Doug Rohrer and Kelli Taylor in Instructional Science, students learned formulas for the volumes of solids either in blocks or mixed together. As the Bjorks' essay summarises, the mixed group solved 63 percent of new problems correctly a week later, against 20 percent for the blocked group.
Interleaving and how it feels. Nate Kornell and Robert Bjork's 2008 study in Psychological Science had people learn painters' styles. Mixing the artists' paintings helped people identify new paintings, yet most participants rated blocking as more effective, even after their own test results showed the opposite.
Generation. The generation effect, set out by Norman Slamecka and Peter Graf in a 1978 paper in the Journal of Experimental Psychology: Human Learning and Memory, is the benefit of producing an answer rather than reading it. The Bjorks put it bluntly: looking up something you could have generated means you "rob yourself of a powerful learning opportunity."
How to add desirable difficulties to a normal study day
Here is how a typical two-hour evening can change without adding time.
Start with a blank page (10 minutes). Before opening yesterday's notes, write everything you remember about the topic. Then check and fill gaps in a different colour.
Mix your problem set (40 minutes). Instead of 15 kinematics problems, do five kinematics, five laws of motion and five work-energy, shuffled. Now each problem asks you to choose the method, which is what the exam asks too.
Attempt before you look (30 minutes). For a new topic, try two problems before reading the worked solution. Getting them wrong is fine; see why mistakes help you learn.
Return to an older topic (20 minutes). Spend time on something from two or three weeks ago, not yesterday. That gap is the spacing.
Change the conditions (20 minutes). Do a few questions standing at a different table, on paper rather than screen, or with a timer. Varying conditions was another of the Bjorks' examples.
When does difficulty stop being desirable?
Watch for signs that you have gone past the useful zone: you cannot start a problem at all, you are stuck on the same step for 20 minutes, or you no longer know what the question is asking. Then step back. Study a worked example, review the concept, and return to mixed practice once the basics are in place. Our piece on John Sweller and the worked example effect explains why beginners need more support.
For the theory behind why forgetting can help, read about Robert Bjork's storage and retrieval strength.
When a mixed set throws up a problem you cannot start, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer. Use it after a real attempt, and you keep the difficulty while getting unstuck.
