"Mixing apples and oranges"
What it is
The idea is to vary the subjects you’re studying. So don’t dwell on one subject too long, study it, study something else, and come back to the first subject later. It is important, however, to not change too quickly: make sure you understand the first subject before moving on to the next. If you go back to the material you already studied, try studying the subject in a different order. Doing so will help to make links between subjects, see the differences and similarities (which they can then think about and elaborate on).
How to do it
For example, you have to learn to compute the volumes of four different geometric shapes. In a so-called blocked-practice condition, you would finish all the problems for one shape before moving on to the next. In interleaved practice, the problems are intermixed, changing from shape to shape.
Does it work, and why?
This strategy is moderately useful, interleaving is mostly useful when learning topics have characteristics that are similar. It is also possible that interleaved practice benefits only those who are already reasonably competent.
Interleaving allows students to practice selecting the correct method and encourages them to compare different kinds of problems. The reason students perform better after interleaving is that interleaved practice helps students to discriminate between the different kinds of problems so that they will be more likely to use the correct solution method for each one.
It also stimulates to actively think about a problem, so that you don’t passively apply one problem-solving strategy to all problems, because you have to think about the correct solving strategy each time. Applying interleaved practice can also conquer the fluency illusion of rereading: when reading or studying one topic too long, it seems familiar and students often overestimate their learning. Interleaving forces to switch between topics, and makes judging own understanding more accurate.
A typical interleaving effect would be that during practice, the performance during practice is better with blocked practice than interleaved practice, but this advantage dramatically reverses on the actual test, such that interleaved practice boosted accuracy by 43%. Interleaving will feel harder than studying the same thing for a long time, but it actually makes for better learning!