Desirable difficulties are actions that appear to work worse in the short-term but work better in the long run.

These include:

Spacing. Practising something ten times in a row vs. ten times spaced out (over hours or days). The latter feels harder but results in more permanent memory.

Variability.  Intuition argues for mastering one thing before moving on to the next, but research suggests otherwise. Variable practice tends to result in better retention and transfer than blocked practice.

Should you re-read or do practice questions? Students overwhelmingly favour re-reading as a learning strategy. However, practice testing is one of the most effective learning methods that has been systematically studied, while re-reading is one of the worst.

What we learn is never erased from our minds. Instead, we forget things as our ability to retrieve them becomes weaker through competition with other memories.

Successful access to hard-to-recall memory boosts retrieval strength more than if the memory was easier to access. It’s as if your brain is saying, “Whoa! That was important and I barely remembered it! Better strengthen that connection.” Easy memory access (say because you just immediately learned it or had the answer in front of you) sends the opposite signal, with correspondingly less benefit.

When you mix practice between two similar ideas or concepts, you’re better able to notice the difference between the two.

This discriminative account in favour of variable practice holds true for many problem-solving skills. Math problems are often taught in a blocked fashion. You learn some problem type and do it repeatedly until you’re good at it. Then, you move on to a different type of problem and repeat the same process. The issue with this blocked approach is that it doesn’t let you practice telling apart the different types of problems because, in each case, it’s obvious.

Cognitive load theory points out that many activities which increase the effort involved in learning tend to result in worse outcomes for typical students. These activities include solving problems you haven’t been taught how to solve, having to split your attention between different sources of information to understand an idea, or having redundant information you need to ignore to get at the answer.