Studies continue to confirm the power of the interleaving effect:
The first signs of interleaving’s promise appeared in the domain of motor skills. One early study, published in 1986, involved training students to learn three types of badminton serves. Compared with blocking, interleaving produced better recall of each serve type and better ability to handle new situations, such as serving from the opposite side of the court. Similar results were later reported for baseball, basketball, and other sports. In 2003, one of the first studies to examine interleaving outside of sports found that using it to train medical students produced more accurate electrocardiogram diagnoses than blocking. In 2008, another widely-cited study found a similar benefit for teaching college students to recognize the painting styles of landscape artists. Even critical thinking skills benefit: in a 2011 study, students trained with the technique made more accurate assessments of complex legal scenarios.
Foreign language studies however suggest that the effectiveness of interleaving comes with an important caveat. When native English speakers used the technique to learn an entirely unfamiliar language, such as to generate English-to-Swahili translations, the results were better, the same, or worse than after blocking. These mixed results imply that learners should have some familiarity with subject materials before interleaving begins (or, the materials should be quickly or easily understood). Otherwise, as appears to be the case for foreign languages, interleaving can sometimes be more confusing than helpful.
Rohrer and his team are the first to implement interleaving in actual classrooms. The location: middle schools in Tampa, Florida. The target skills: algebra and geometry.
The three-month study involved teaching 7th graders slope and graph problems. Weekly lessons, given by teachers, were largely unchanged from standard practice. Weekly homework worksheets, however, featured an interleaved or blocked design. When interleaved, both old and new problems of different types were mixed together. Of the nine participating classes, five used interleaving for slope problems and blocking for graph problems; the reverse occurred in the remaining four. Five days after the last lesson, each class held a review session for all students. A surprise final test occurred one day or one month later. The result? When the test was one day later, scores were 25 percent better for problems trained with interleaving; at one month later, the interleaving advantage grew to 76 percent.
Researchers are now working to understand why interleaving yields such impressive results. One prominent explanation is that it improves the brain’s ability to tell apart, or discriminate, between concepts. With blocking, once you know what solution to use, or movement to execute, the hard part is over. With interleaving, each practice attempt is different from the last, so rote responses don’t work. Instead, your brain must continuously focus on searching for different solutions. That process can improve your ability to learn critical features of skills and concepts, which then better enables you to select and execute the correct response.
A second explanation is that interleaving strengthens memory associations. With blocking, a single strategy, temporarily held in short-term memory, is sufficient. That’s not the case with interleaving — the correct solution changes from one practice attempt to the next. As a result, your brain is continually engaged at retrieving different responses and bringing them into short-term memory. Repeating that process can reinforce neural connections between different tasks and correct responses, which enhances learning.
Despite these relative advantages, interleaving remains mostly unknown and unused. Consider the example of grade school math. Out of all the math textbooks used in the U.S. today, all but one type — the Saxon series — uses blocked practice.