Does interleaving, or mixed practice, really help you study?

When you study math, you probably do ten derivative exercises, then ten on sequences, then ten on probability. That is grouped practice, also called blocked practice. Interleaving does the opposite. You mix the types, one derivative, one sequence, one probability, then start again in another order.

Grouped practice is comfortable. Since you already know which method to use, you do not have to ask yourself anything, you just apply it. With mixing, you first have to recognise what kind of problem you are facing, then choose the right method. That takes more effort in the moment, and the session feels less smooth. The idea behind interleaving is precisely that this extra effort is what pays off in the long run. The next sections show what researchers measured, without stretching beyond what they tested.

Rohrer and Taylor (2007) compared two ways of practising math problems. In one version, the problems were grouped by type. In the other, the types were mixed. On the delayed test, the mixed version gave better results than the grouped one (Rohrer and Taylor, 2007). So the result shows up when you test later, not only at the end of the session.

That detail matters. At the end of a session, grouped practice can give you the feeling that you understood everything, because you string together successes. The delayed test shows something else, what remains when the method is no longer hinted at by the previous exercise. This echoes a theme found in How to study effectively, according to the research, what feels smooth is not always what sticks best. The source gives no usable number here, and we will not invent one. Keep the direction of the effect, and test it on your own exercises.

In math, the study by Rohrer and Taylor covers exactly this ground, so you can borrow from it quite directly. For finals, instead of doing ten exercises on the same topic, build a sheet of six problems, with a derivative to study, a sequence to express, a conditional probability, and so on. Do it without looking at your course, then correct it.

In physics, the principle is the same. Mix, for example, a kinematics problem, an energy problem, an optics problem and an electric circuit problem. Before calculating, write one line saying what type of problem it is and which law you will use. That recognition step is what mixing forces you to do, and what grouping lets you skip. Do the same with chemistry chapters that look alike.

For languages, interleaving transposes well. In Spanish, if you work on ser and estar for twenty minutes, then the preterite for another twenty, you always know which rule to apply. Mix the two in the same set of sentences, with por and para too, and you have to choose every time.

The same goes for English in your AP or finals prep. Instead of doing one page of phrasal verbs, then one page of past tenses, build a mixed set with a little of each, and add vocabulary. You can also alternate, within the same session, reading comprehension and grammar. The goal is not to make practice painful. It is to make you recognise the situation before you answer, as you will have to on exam day, when nobody tells you which chapter is up.

Dunlosky, Rawson, Marsh, Nathan and Willingham (2013) reviewed ten study techniques. Interleaved practice is rated as moderate utility there, behind practice testing and distributed practice, which are rated high (Dunlosky et al., 2013). That means interleaving is worth it, but it does not come before the basics. If you have to choose where to start, start with testing yourself and spacing.

Bjork, Dunlosky and Kornell (2013) also discuss desirable difficulties, efforts that slow you down in the moment and can help later (Bjork et al., 2013). Mixing is a good example in spirit. The table below reproduces the Dunlosky ranking for a few techniques, to keep proportions in mind. Treat interleaving as one useful tool among several, best added once testing yourself and spacing are already in your routine.