Interleaving

What it is

Interleaving means shuffling different kinds of problems together rather than practising one type to exhaustion before moving on. A blocked session does twenty quadratic equations, then twenty on circles. An interleaved session mixes them, so you never know which method a question needs until you look at it. That extra step — deciding which approach applies — is exactly the step the exam requires and blocked practice removes.

How to do it

• Mix question types from different chapters into one practice set.
• Do not let yourself know in advance which method a question needs.
• When revising, alternate between subjects in a session rather than blocking whole days.
• Use mixed past papers rather than chapter-end exercises, which are blocked by design.
• Accept a lower success rate during practice; it will be higher when it counts.

How good is the evidence

Studies on maths and physics learning consistently find interleaved practice produces worse performance during practice and better performance on later tests — sometimes dramatically so. The effect is strongest where problems look similar but need different methods, which describes most competitive exam mathematics. It is less clearly useful for material where there is nothing to choose between, such as vocabulary.

Where people go wrong

• Interleaving before you can do any of the individual methods. Learn the method first, then mix.
• Concluding it is not working because practice scores drop. That drop is the mechanism.
• Mixing material that is unrelated enough that no choosing is involved.

Chapter-end exercises quietly do you a disservice: because every question comes from the chapter you just read, you never practise the hardest part of an exam, which is working out what kind of question you are looking at.

This is why students who can solve every problem in the textbook still freeze in the exam hall. They learned the methods but never practised choosing between them.