**“I get 100% on the homework, but I bomb the exams.”**

I can’t tell you how many times I’ve heard this statement from students and parents. Many students and parents attribute the problem to “test anxiety”, or to the teacher not giving problems on the exam that were similar to the assigned homework, or some other excuse. Most of the time, however, the problem stems from how the student prepare for the tests.

If you use Interleaving, I guarantee your test grades will improve dramatically. If you combine Interleaving with Active Recall and Spaced Repetition, you will quickly be on your way to the top of the class. These three methods combined – Active Recall, Spaced Repetition, and Interleaving – are the trifecta for success in academics. If you change your study habits to incorporate these three techniques, you will simultaneously cut your study time and improve your grades. Dramatically.

**What is Interleaving?**

Interleaving is the method of mixing up problem types when you practice or study for a test, instead of practicing all one type of problem, and then moving on to the next, and then the next, and so on.

For example, if you have taken a geometry class you probably remember that there are bunch of different formulas for calculating the areas of various shapes: a triangle is one-half base times height, a square is edge-squared, a rectangle is length times width, a regular polygon is one-half apothem times perimeter (might not remember that one!). A typical geometry textbook will cover one shape and one formula at a time. Section 1: Triangles. Section 2: Squares. Section 3: Rectangles. And so on. And maybe at the end of the chapter will be a chapter test that has all of the different problem types, but they are still in the same order as they were presented in the chapter. This is called “blocked” studying. It just means you do all of one problem type before moving on to the next.

There is a huge problem with this type of learning: YOUR BRAIN GOES ON AUTOPILOT AND YOU DON’T LEARN HOW TO RECOGNIZE THE TYPE OF PROBLEM YOU’RE SOLVING. You just get into the groove and apply the same concept to problem after problem without even thinking about what you’re really doing. Sure, some problems might be set up a bit differently, but all of the problems follow the same general pattern and use the same equation, so your brain doesn’t have to do any work to identify what type of problem it is in the first place. But when you get the test, all of the problems are mixed up and you brain says, “Oh, crap! I don’t know how to start this problem!” Then you freak out a little bit, and bomb the test.

Must be test anxiety, right? Wrong. The problem is that you didn’t teach your brain how to RECOGNIZE what kind of problem it was, so you don’t know how to get started. Maybe you don’t remember the formula for the area of triangle because your brain went on autopilot when you were working all those triangle problems in a row, so you neglected to remember the most important thing from the section – the formula for the area of a triangle! In working with students I’ve notice that about 80% of the troubles they have are in getting the problem started. As soon as they get it started, and have identified the correct formula or concept to apply, then solving the rest of the problem usually falls into place.

When students “bomb” an exam and then review the answers afterwards they usually say things like “Why didn’t I see that?” or “That was so easy!” or “How could I be so stupid!” or “That was an easier problem than I thought…” or “That was so simple.” Of course, because if they had just been able to RECOGNIZE what kind of problem it was, and could get past the first step, they probably would have gotten the right answer.

**How Do I Use Interleaving?**

With the layout of most textbooks it can be tricky to incorporate Interleaving into your study routine. However, most textbooks now have a chunk of problems, or even a quiz, at the end of each section reserved for cumulative review, that is, problems mixed in from all of previous chapters and sections, or at least from the current chapter. If your teacher doesn’t assign these, do them. If your book does not have such a thing, then flip to the end of the chapter review section (almost every textbook has this) and work problems randomly, not in order. For example, if there are 20 problems, work them in an order like this: 3, 12, 1, 8, 19, 17, 4, 9, 5, etc. If you have a homework assignment, try to estimate how much space your each of your problems will take, write down all the numbers, and then do them in a random order. Sure, your homework might not end up all neat, tidy, and organized, but its better to have a homework assignment that’s not perfectly in order than to get a D on your test. If you can’t solve one of the tougher problems that are towards the end, you can always jump to an easier problem and then come back to it. Oftentimes, working an easier problem will give you the insight you need to solve the tougher one.

**Why Does Interleaving Work Better Than Working Problems in Order?**

When you use Interleaving, you train your brain to recognize the different TYPES of problems. If all the problems are mixed up, the first question your brain has to answer is “What kind of problem is this?” The second question your brain has to answer is “How do I get this one started?” If you work problems in order from beginning to end, you can skip these two questions, your brain never learns how to recognize the problems and get them started, and then you blank on the test because you don’t know what to do. That’s why students get the answer key later and wonder how they missed so many “easy” problems.

**Scientific Studies Show That Interleaving Can Double or Triple Test Scores**

There are now several good studies showing that Interleaving works astoundingly well. The first study that isolated the effects of Interleaving was a study done on elementary school students in Florida. The students were taught how calculate the number of faces, edges, corners, and angles of different prisms based on the number of base edges. For those of you who need a refresher, prisms are three-dimensional objects with the same top and bottom, like a Toblerone box (triangular prism), cardboard box (rectangular prism), or more complicated shapes like the hexagonal prism above.

The students were split into two groups. Group 1 used the traditional “block” study method where they learned how to calculate the numbers of faces and did a bunch of those problems, then they learned how to calculate the number of edges and did a bunch of those problems, then corners, then angles. Group 2 used Interleaving, where the problems asking for the number of faces, edges, corners, and angles were all mixed up. On Day 1, the two groups spent the exact same amount of time working practice problems. The students using the traditional “block” method got more problems right when they were practicing on Day 1 than the Interleaving students got correct (98% vs. 79%). OMG, Interleaving doesn’t work, right? WRONG! On Day 2, both groups took a test on problems they learned how to solve just a day earlier, but all the problems were mixed up, like they are on a real test. The average score for the kids in Group 1 (Normal study method) was 38%. The average score for kids in Group 2 (Interleaving) was 77%!. The Interleaving students MORE THAN DOUBLED the other kids’ scores!

The studies are pretty consistent – when students are learning new material, Interleaving dramatically improves scores, regardless of the subject area. And oftentimes, the Interleaving group will double or triple the test scores of the control group. Assuming that you are already putting hard work to study the normal way, if you start to use interleaving, it’s very likely that your grades will move up at least a couple of grade levels. From an F to a B or C. From a C to an A or B. And the best part is that you don’t need to study longer or harder, just use interleaving!

What’s very interesting about these studies is that the group of students studying the normal way THINK they are learning the material better, but when they are tested later, they ALWAYS perform worse than if they had used Interleaving.

These results are even consistent for teaching people physical activities, like badminton serves and hitting different types of baseball pitches. If you mix up the types of activities (Interleaving) the people always learn the activity faster and better, even though they THINK they are learning faster and better using the normal method of doing one type, and then moving on to the next type. So the moral of these stories is to not trust your brain when it tells you that studying one thing at a time works best!

**Why Interleaving Works Better**

Interleaving works better than traditional studying because your brain learns how to discriminate between the different types of problems, concepts, or activities more quickly. Your brain is REALLY good at spotting differences (discriminating) between types of problems, concepts, or activities, and it uses that ability to help it learn faster, whether its figuring out which formula to use for different shapes, or learning how to hit a curveball versus a fastball versus a change-up. You don’t want every problem to look the same to you when you get the test, right? So learn how to spot the differences by using Interleaving and you won’t have to worry about not getting stuck on the test.

**So here’s the take-home message:**

Interleaving helps your brain recognize different types of problems, and get them started. When you take a test it’s blank, and the problems are usually all mixed up. You need to work practice problems out of order to train your brain how to discriminate between the different types of problems so you don’t blank on the test!

**References**

- Taylor K, Rohrer D. The effects of interleaved practice. Applied Cognitive Psychology. 24: 837-848. 2010.
- Birnbaum et. al. Why interleaving enhances inductive learning: the roles of discrimination and retrieval. Mem. Cognit. 41(3):392-402. 2013.
- Hall EG, Domingues DA, Cavazos R. Contextual interference effects with skilled baseball players. Perceptual and Motor Skills. 78:835-841. 1994.

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