Interleave It

The steps

1. Start once each type is accurate on its own. Teach a new problem type and let students practise it in a block first. Move to mixed sets once most of the class can solve each type unaided. A short blocked warm-up will show you whether they are ready.
2. Build the set from types students confuse. Choose three or four problem types for a single set of eight to ten questions. Pick types that look alike at first glance, such as area and perimeter, or mean and median. To save time, reuse an old worksheet and swap half of its questions for earlier types.
3. Order the questions so neighbours need different methods. Shuffle the questions until no two consecutive ones use the same method. Then check the finished set: if a student could guess the method without reading the question, reshuffle.
4. Tell the class why the set is mixed. Give the reason before the first mixed set, and repeat it whenever students protest.
   These questions are mixed on purpose. In the exam, nobody tells you which method to use, so today you practise choosing it. It will feel harder than a normal page. Stick with it. Choosing the method is a skill, and this is how you train it.
5. Ask for the method before the working. Ask students to write the method name in the margin before they solve each question. One word is enough, such as "perimeter" or "factorise". The margin note shows both you and them where the choosing goes wrong.
6. Mark the choice as well as the answer. When you go over the set, ask which method each question needed and how students could tell. Name the clue in the wording, such as "around the edge" pointing to perimeter. Spend longest on the questions where several students chose the wrong method.

How you’ll know it’s working

• Method words start appearing in the margins before the working begins, without a reminder from you.
• The gap between blocked and mixed scores narrows. A class scoring nine out of ten on blocked pages and five on mixed pages in October may reach seven or eight on mixed pages by December.
• Questions like "is this an area one?" fade during mixed practice, and students begin to point to the wording that gives the type away.
• Marking a mock reveals fewer wrong-method answers and fewer blanks, especially on questions drawn from earlier units.

Weigh it up

Why it’s worth it

• It rehearses the choice that exams demand. Papers mix their questions, so a student who has only practised in blocks meets method-choosing for the first time in the mock.
• The measured gains are large. In Rohrer and colleagues' randomised trial, classes that mixed their practice for four months scored 61% on a test a month later, while blocked classes scored 38%.
• Every mixed set doubles as spaced review. Earlier types keep returning within ordinary practice, so the previous topic stays live while the new one is taught.
• It shows you who cannot tell the types apart. Blocked pages hide this entirely, while a margin word for each question tells you which pairs to reteach, and to whom.
• It needs no new resources. The questions already exist in your textbook and worksheets; the work lies in reordering them, and a set built once serves every year that follows.

The costs

• Lessons feel less successful while the technique beds in. Scores drop, more hands go up, and the class gets through fewer questions, because each one now begins with a decision.
• The reordering falls to you. Published exercises arrive blocked, so building mixed sets takes planning time until your file of them has grown.
• The early dip lands on students too. Confidence takes a knock for a few weeks, and in a room where mistakes feel risky, a mixed set can read as a trap.