One of the sabbatical projects I undertook while writing this book was learning Spanish, a task I had attempted but then abandoned during graduate school many years ago. Although ostensibly I embarked upon this more recent endeavor in preparation for some upcoming travel to Latin America, in truth I count studying and learning languages as one of my favorite pastimes. (I will pause here to allow you to savor the fact that you probably lead a more socially engaging life than I do.) My school language learning includes Latin, Greek, and French, but I have also made independent efforts to learn Spanish, Italian, and Gaelic at various points in my life. I wouldn't get very far conversing with the residents of ancient Rome or the native Gaelic speakers of Ireland's Aran Islands, but some aspects of all of those languages have stayed with me. Because I have spent so many hours memorizing foreign language vocabulary and studying grammatical structures of other languages, the process has become one with which I am comfortable and familiar. Even though it had been a few years since I launched a full and earnest effort to master a language, I assumed that my Spanish study would come easily enough as long as I put in the requisite time and effort.
My course of study began with an online program that I hoped would guide me through the early stages of review and basic acquisition, after which I would expand to other activities like reading novels or watching television shows in Spanish. For a month or two, I spent 15 minutes every day online listening to brief sentences in Spanish, repeating them back into the computer microphone, translating Spanish sentences into English and vice versa, and taking occasional quizzes. The individual lessons of the program were broken into segments that took about five minutes to complete, which meant I could complete three new ones every day. Each new segment included a small measure of reinforcement of the material I had already learned, but the lessons focused mostly on acquiring new vocabulary or identifying new rules of grammar or syntax. A month into following this schedule of three new lessons per day, I found myself increasingly forgetting vocabulary I had learned just days before and regularly failing the occasional timed quizzes I took. I would learn the word for “tie” (corbata), and then the next day mix it up with the word for “belt” (cinturon). A few weeks later, one of those words would pop up for review, and I wouldn't remember either of them. My progress in understanding the language seemed very slow in contrast to my previous experiences. Every lesson felt like a new struggle to me. I assumed that the program—which multiple people had recommended to me—had been constructed by folks who knew something about language acquisition and that therefore the problem must lie with me. Either I was not spending enough time on my study, or my aging brain was no longer as adept as it once was in learning languages. All I could think to do was redouble my time and effort.
Then one day I noticed a tab on the home page labeled Strengthen Skills. I clicked on it, and it took me through a 5-minute review session that mimicked the activities of the normal lessons but introduced nothing new and contained only vocabulary and sentence structures that I had already covered. At first I found it frustrating to complete these exercises since I was stumbling over vocabulary that I had supposedly learned already, but I began gradually incorporating more and more of these review sessions into my learning time. Eventually my routine shifted from three new lessons per day to one new lesson per day and two review sessions. Within weeks of making this change I felt the budding mastery that had been eluding me begin to emerge; I began regularly acing the timed quizzes and feeling much more comfortable with my pronunciation efforts. Of course, my progress through new material slowed down, but this seemed like a small price to pay for a much more firm understanding of that new material through these repeated review sessions. Perhaps most important, what had felt like a painful struggle to me now became enjoyable.
The learning principle that helps explain this improvement in my language acquisition skills is called interleaving, and it involves two related activities that promote high levels of long-term retention: (a) spacing out learning sessions over time; and (b) mixing up your practice of skills you are seeking to develop. A study conducted almost 30 years ago on French language acquisition in an American high school provides our first illustration of this principle (Bloom and Shuell 1981). The researchers divided around 50 students into two groups and charged each group with learning 20 new French vocabulary words in different ways. The first group had a single 30-minute session in which they studied the new vocabulary words and completed three separate tasks on them, such as filling in the French word after receiving the English equivalent. The second group had the exact same length of study time and the exact same set of written exercises, but they were separated into three 10-minute study periods over the course of 3 consecutive days. The contrast between these two methods is usually described in the literature as massed versus spaced (or sometimes distributed) learning. In massed learning, students focus entirely on one skill or set of material until they have mastered it; in distributed practice, students space out their learning sessions over time. At the end of the study periods for both groups in this experiment, the students were given a vocabulary test on the words; both groups averaged about 16 of 20 words correct. This finding will appear again and again in the literature; for short-term retention, massed practice can be as effective (and sometimes more effective) than distributed practice. The researchers then returned to the classroom a week later, without any prior warning to the students, and tested them on the vocabulary again. This time the results diverged sharply: the massed practice students remembered around 11 of the vocabulary words, whereas the spaced practice students remembered around 15. Remember that both groups had the same total learning time and completed the same tasks; only the spacing of their learning activities differed.
A substantial body of research has demonstrated the power of spaced learning. Benedict Carey wrote in How We Learn that “nothing in learning science comes close in terms of immediate, significant, and reliable improvements to learning” (p. 76). The theory that explains the power of spaced learning stems at least in part from what we have learned about the importance of retrieval practice. One of the challenges to our memories is the ability to pull desired information from our long-term memories when we need it. The more times we practice drawing specific skills or information from our long-term memory, the better we get at it. When we engage in massed learning exercises, focusing on one set of content repeatedly, we never have to access the learned material from the deeper recesses of our long-term memory. By contrast, if we use spaced learning to allow some time for the forgetting of learned material to set in, we are forced to draw material from our longer-term memory when we return to it. Spacing out learning thus forces us to engage at least partially in memory retrieval.
That forced cycle of forgetting and retrieving is only half the explanation for the power of spaced learning. As the authors of Make It Stick explain, the time that intervenes between spaced learning sessions also allows our minds to better organize and solidify what we are studying:
Embedding new learning in long-term memory requires a process of consolidation, in which memory traces (the brain's representations of new learning) are strengthened, given meaning, and connected to prior knowledge—a process that unfolds over hours and may take several days. Rapid-fire practice leans on short-term memory. Durable learning, however, requires time for mental rehearsal… Hence, spaced practice works better. The increased effort to retrieve the learning after a little forgetting has the effect of retriggering consolidation, further strengthening memory. (Brown, Roediger, and McDaniel 2014, p. 49)
Our brains need time to undertake the processes of encoding, consolidating, and organizing newly learned material, and the gaps between spaced learning sessions allow it that time. If you have ever slogged your way through some difficult learning exercise, left it in frustration, and then—hours or days later—returned to it with a mysteriously firmer grasp of it than you had previously, you have experienced the phenomenon described by the authors of Make It Stick. I remember this happening to me time and time again when I played the piano more regularly. I would stand up from a practice session convinced that I would never master some difficult passage and then sit down the next day and find that it had mysteriously become much easier than it was the day before.
The implications of this principle are clear enough for both learners and teachers: we should help students space out their learning both in how we design our courses and in how we encourage them to study. However, we can help our students even further if we consider spaced learning as one aspect of interleaving, a broader approach to helping our students learn. Interleaving refers to the practice of spending some time learning one thing and then pausing to concentrate on learning a second thing before having quite mastered that first thing, and then returning to the first thing, and then moving onto a third thing, and then returning to the second thing, and so forth. In short, it involves the process of both spacing and mixing learning activities—the spacing happening by virtue of the mixing. As a simple example, suppose that I want to teach my students four major concepts: A, B, C, and D. Standard educational practice would have us spend 3 weeks on concept A, 3 weeks on concept B, 3 weeks on concept C, and 3 weeks on concept D. I might have a unit test at the end of each 3-week period and then a cumulative final exam. An interleaved approach would look quite different. I might instead spend the first 2 weeks introducing all four concepts, giving students a brief overview of them each. Then for 2 weeks I would dig in on concept A (followed by a quiz) and another 2 weeks on concept B (another quiz). We would pause at that point and review both concepts together and then take an exam. We would repeat the pattern with concepts C and D and then review all four concepts together for a couple of weeks before we would take that final exam. At the end of Week 3 my students are not going to know concept A as well as the students who spent 3 weeks learning it in the standard form. According to all of the research we have on interleaving, though, they are going to know it much better than the students in the massed example at the end of the semester—and, more importantly, after they have left the course.
As you are reading this, you are perhaps thinking to yourself that this all sounds very messy and might even provoke frustration from you and your students since it would be much neater and cleaner to march your way through the concepts in order. Indeed it would, and the research on interleaving confirms what you suspect: learners often find it frustrating. I experienced this frustration myself when I began using that Strengthen Skills tab in my language-learning program and found that I didn't know nearly as much as I thought I did. Research also tells us that massed practice works very effectively for short-term learning, which is why students like it and why they can often perform well on exams when they engage in massed learning exercises like cramming. Jeanette Norden, a neuroscientist who has taught medical students at Vanderbilt University for several decades now using an interleaved approach, compares teaching this way to a form of spiraling. The first time you approach the material, you are making a single spiral at the bottom level. The next time you return to it you are circling back through the material but at a slightly higher level. Spiraling can feel frustrating to the learner because you are, in a sense, going around in circles. However, you are also moving upward with each spiral, adding new layers of learning every time you push back through the material. The effectiveness of Norden's approach to medical education, which has been seen as unconventional by many of her peers, is evident enough in the results. One of the teachers profiled in Ken Bain's book What the Best College Teachers Do, Norden has “won every award for teaching granted by the medical school and selected by the students—some of the awards more times than the university will allow” (p. 6). Her students have consistently registered outstanding performances on the neuroscience portion of the national Medical Licensing Exam. Norden's experiences suggest that a little discomfort during the interleaved learning process can have major payoffs in the long run.
Laboratory studies that have been conducted comparing massed versus interleaved learning likewise leave little doubt that interleaving trumps massing for long-term retention by a very wide margin. Consider a frequently cited study in this area in which students were tasked with the challenge of learning to solve math problems involving different geometric shapes (Rohrer and Taylor 2007). In this experiment the students all received brief tutorials on how to calculate the volume of four different geometric shapes, including seeing a worked example, and then were asked to solve 16 different problems that required them to use what they had learned. The tutorials and problem-solving sessions took place on two separate occasions, a week apart. In one group, the Blockers, the students had a tutorial and then solved four problems on it; had a second tutorial and then solved four problems on it; and so on. In the other group, the Mixers, the students received all four tutorials at once and then were given the 16 problems in random order. While the students were working on the problems, the Blockers performed better. During the first learning session, for example, the Blockers solved 89 percent of the problems correctly; the Mixers solved only 60 percent of them correctly. One week after the practice sessions were completed, the groups returned to the laboratory and were given a new set of eight problems, in random order, two on each of the four shapes. The difference between the groups is astonishing: the success rate of the Blockers dropped down to 20 percent, whereas the success rate of the Mixers improved to 63 percent.
In this experiment, both groups engaged in spaced learning; they had two distinct sessions, separated by a week, and the test was given a week after that. We don't have a comparison group in which, say, students completed 32 problems in one massed session instead of the two separate sessions spaced a week apart, but we can assume from previous research on massed versus spaced learning that both the Blockers and Mixers would have outperformed that group. So given that both groups engaged in spaced learning, this experiment particularly highlights the benefits of interleaved learning: mixing your study or practice as well as spacing it. The authors of the study present this brief explanation for why they believe the Mixers so definitively outperformed the Blockers: “The superior test performance after mixed practice is, in our view, attributed to the fact that students in this condition were required to know not only how to solve each kind of problem but also which procedure (i.e., formula) was appropriate for each kind of problem (i.e., solid)” (Rohrer and Taylor 2007, pp. 493–494). In other words, the Mixers had to learn not only how to plug and chug the mathematical equations but also how to identify the type of problem they were seeing and to select the formula that would work for that problem. They could not work on autopilot, as a student might do in a class session in which he learns Formula A and then applies it to Problem Type A for an extended period of time, knowing that Formula A will always work for Problem Type A, and every problem he will see in the session will be Problem Type A. Hence, “a significant advantage of interleaving and variation,” argued the authors of Make It Stick, “is that they help us learn better how to assess context and discriminate between problems, selecting and applying the correct solution from a range of possibilities” (Brown, Roediger, and McDaniel 2014, p. 53). And this is important, as they note, because real-world performance contexts require this skill: in life, as on final exams, “problems and opportunities come at us unpredictably, out of sequence. For our learning to have practical value, we must be adept at discerning ‘What kind of problem is this?’ so we can select and apply an appropriate solution” (p. 53). Blocked learning does not require students to make such choices about which learned skill to apply in which context.
This explanation for the limits of blocked practice and the benefits of interleaving points to a deep and fundamental challenge that all learners face: transferring learning from the original context in which we encounter it into novel or unfamiliar contexts. A great deal of research has been done in this area, and the consensus has been that fundamentally we are not very good at doing this. “Transfer,” wrote Michelle Miller in Minds Online, “is remarkably hard to achieve, a particularly unsettling fact given that it is also such a high-stakes issue; after all, an education that doesn't transfer isn't worth much” (Miller 2014, p. 130). We learn in specific contexts, those concepts become familiar to us, and we have trouble transferring that learning into other contexts. So students who learn a specific writing skill in my composition class never think to apply it to the history paper they are writing; students who master the scientific method in biology don't think to apply it in the psychology course they are taking. Blocked study or practice deepens our association between a learned skill or concept and the specific context in which we learned it; interleaved learning, by contrast, forces us into frequent transfers of information and skills across contexts, which helps us develop the ability to recognize when a learned skill might apply in a new context. The students in the math experiment, when they were taking that final test, were faced with novel problems in random order. The students who had engaged in mixed practice were much more effective than the blockers at rooting around in their memory for the full set of skills they had learned and applying them in this new context. Cultivating the ability of our students to draw from memory and apply learned concepts or skills to new situations is, as Susan Ambrose and her colleagues argued, the “central goal of education” (Ambrose, Bridges, DiPietro, Lovett, Norman 2010, p. 108). Interleaved learning facilitates that goal more effectively than massed learning.
Before we push into the models for interleaving in higher education pedagogy, though, I have to offer one essential clarification. In the initial learning phase, blocked study or practice is not a bad thing—and for some types of learning tasks it might even be a necessary thing. A recent study by two psychologists at Iowa State University compared the effects of blocked and interleaved practice on students who were learning the pronunciations of French words (Carpenter and Mueller 2013). Over the course of several experiments, they found consistently that the students who had the opportunity to repeat the pronunciation of familiar words over and over again in blocked fashion outperformed those who learned those pronunciations in interleaved fashion. Their survey of the literature also points to one or two experiments in which blocked learners have outperformed interleavers on certain types of tasks (although they acknowledge that the bulk of published studies supports interleaving). The final recommendation they make in their conclusion, though, seems like an eminently sensible one: “Rather than using a schedule that is exclusively blocked or interleaved, it may be more advantageous to start with a blocked schedule and then transition to interleaving” (p. 680). Blocked study or practice, it seems to me, is an appropriate first step for any learning activity. As Benedict Carey put it, “It's not that repetitive [or massed] practice is bad. We all need a certain amount of it to become familiar with any new skill or material” (Carey, 2014a, p. 157). Indeed, I suspect most of us introduce new material to our students, or learn it ourselves, by blocking the study or practice of it. We have to begin the learning process by spending some concentrated time or effort on the task. The argument I am making here is not to eliminate blocked practice but to use interleaving to require students to return continuously, in different contexts, to material they have learned already. Blocking on its own is not a problem; blocking without interleaving—otherwise known as cramming—produces wonderful short-term retention but will leave our students without the long-term retention that will enable them to extend their learning beyond the final exam.
The application of the small teaching philosophy to the learning principle of interleaving occurs less in the form of specific in-class activities than in the form of tweaks or modifications to your course design. So the following models focus less on discipline-based examples and more on how to achieve interleaving in three different contexts: (a) through the design of your assessment plan; (b) through the organization of your class time; and (c) through the use of an online course management system.
If you combine the research we have considered on the importance of retrieval practice and the research on the power of interleaving, the implication is an obvious one: all major exams in your course should be cumulative. Research on learning supports this implication. In one recent study of cumulative versus noncumulative exams in psychology courses, researchers analyzed the scores of students in a cluster of psychology courses on a postcourse assessment; students who had taken a cumulative final exam scored substantially higher on the postcourse assessment than those who had taken noncumulative finals (Khanna, Brack, and Finken 2013). In some of the courses, the positive learning effects of the cumulative final exam persisted as long as 18 months after the completion of the course.
More generally, every major assignment should require students to draw—at least a little bit—on information or concepts or skills they have learned in previous units. This does not have to mean that the third exam of the course must be divided into three parts, one on each of the first three units. It may be that the third exam focuses primarily on the third unit, with two-thirds or three-quarters of the tested material deriving from that section of the course. But the final third or quarter should require students to return to material from earlier parts of the course. You can even accomplish this in a less obvious way by giving assignments or asking exam questions that require students to compare current content or skills with previously learned material. In my literature survey course, which divides into four units over the course of the semester, each exam requires students to answer three or four large essay questions. After the first exam, one of those questions always requires them to compare an author or event or trend from the current period with one from a previous period. They are warned about this, which gives me an opportunity to remind them about the importance of continually returning to the authors and ideas we have already discussed.
Quizzes represent another excellent opportunity to leverage the power of interleaving in your courses. Select some reasonable percentage of your quizzes that will be devoted to previously covered material, and stick with it throughout the semester. If you give 10-question multiple-choice quizzes on a weekly basis, set aside two questions for previously learned material. If you give one-question writing-based quizzes, as I do, ensure that every third or fourth quiz requires students to return to previously learned material. This is especially important to do if you make your exams cumulative, as of course you should. The more students are asked to return to previous material on the quizzes, the better they will be prepared to do so on the exams. Overall, you should consider your total package of quizzes and exams as the ideal tool for continuously reinforcing learned material from the first week of the semester to the last. If you don't give a cumulative final exam, you are essentially conveying to students that what they learned in the first weeks of the semester doesn't matter anymore. If you do give a cumulative final exam but not cumulative mid-terms or quizzes that test them on previously learned material, you are not giving them the kind of help they really need to solidify and enhance their early-semester learning on the cumulative exams.
I would be omitting a truth you would quickly discover on your own if I did not reiterate at this point that students might not respond with unbridled enthusiasm (at least initially) to these kinds of modifications to your assessment plan. Just as I felt frustration when I first began to test myself on previously learned Spanish vocabulary and quickly realized how little of it I remembered, your students might feel initial frustration at the expectation that everything they have learned remains on the table for all of their quizzes and exams. Maryellen Weimer, in a post on cumulative exams on the website Faculty Focus (Weimer 2015), offered some excellent suggestions for helping reconcile students to cumulative exams, all of which sit perfectly within the framework of small teaching activities, requiring just a small investment of class time:
Strategies like these give you the opportunity to announce to students from the beginning of the semester that all learning in the course will be cumulative, and they give your students the help they need in preparing to succeed on cumulative exams.
You might remember that one of the suggestions made in the first chapter was to use opening questions that required students to remind you what you did in the previous class, week, or unit of the course. That recommendation was made in the service of giving your students practice in retrieving previously taught information. However, now you will recognize that it also contributes to an interleaved approach to the material, since Wednesday's class will induce them to return to Monday's material, and so on. Asking questions about previous material (e.g., in the form of the test questions recommended by Weimer 2015) would constitute a very low-level form of interleaving and can be easily supplemented or enhanced by more substantive exercises. For example, teachers of science, technology, engineering, and mathematics (STEM) classes, which include problem solving as one of the major learning activities, often ask students to complete homework problems prior to class and then open the next class with a brief review of those problems or the opportunity for students to ask questions about them and clarify answers. In most cases, following this brief review, the class will then move on to the new topic and leave those problems behind. A very slight modification to this traditional strategy—the essence of small teaching—would help provide that interleaving boost. Assume for a moment that the students worked on problems the previous evening and now are sitting in your class ready to review them and ask a few questions. You spend those first few minutes on questions and review as usual. Then, before moving on to the new material for that day's class, you give them one more new problem to complete right there in class. Make it a quick one so it doesn't eat up too much class time, but doing this will give them one more (distributed) opportunity to practice the problem-solving skill that you introduced in the last class period.
Thinking more generally about how you divide up your course material, you might consider a change that requires no new techniques or strategies—just a small (teaching) shift in how you introduce the material in chronological terms. A common organizational structure in any problem-based course would be to spend the bulk of the class working on a single idea or problem type and then to give students a little time at the end to work independently on a problem of that type. This is a sensible enough structure, and if you are indeed giving them time at the end of class to work on problems you are doing something good already (as we shall see in Chapter 5). An alternative to this structure would be to divide the class session into two halves, each of which focuses on a different topic, with the problem-solving session coming in the middle. So Monday's class might begin with a review of the material covered on Friday and addressed in the homework, followed by a 10-minute problem-solving session—which will have the added benefit of breaking the class at the midway point and renewing student attention for the second half of class. In that second half, you introduce new material. They do homework on that material for Wednesday, which begins with a review and then problem-solving session, and so on. The contrast between these two approaches would look like this:
| Blocked Class Sessions | Interleaved Class Sessions |
| Monday: Topic A, Problem-Solving Session | Topic A, Problem-Solving Session, Topic B |
| Wednesday: Topic B, Problem-Solving Session | Topic B, Problem-Solving Session, Topic C |
| Friday: Topic C, Problem-Solving Session, Quiz | Topic C, Problem-Solving Session, Review or Quiz |
The potential objection you might have to this approach would be that Topic A takes a full 40 or 60 minutes to explain and introduce. Remember, though, that asking students to struggle a little bit with material that they have not fully mastered will help draw on the learning power of prediction. In addition, interleaving can produce initial feelings of frustration in learners, precisely because they don't have the opportunity for full mastery before moving to the next thing. A little discomfort on either your part or the part of the students is not a bad thing as long as you can ultimately get beyond it and get into deeper learning.
In Minds Online, Miller argued that online learning environments provide an ideal tool for creating interleaved learning experiences for our students. As we saw in the first chapter, she recommended the strategy of “staggered online deadlines that ensure spaced rather than massed work” (Miller 2014, p. 109). In a fully online course, she suggested that instructors “set up a recurring weekly schedule where each kind of work (discussion, quizzing, homework, any higher-stakes assignments such as major exams or papers) is due on a different day. You can set things up so that students are welcome to work ahead, but can't fall behind; some will manage to mass their work anyway by turning everything in extremely early, but those students are exceedingly rare” (p. 109). Exceedingly. For the majority of our students the use of such staggered deadlines will have the desired effect, especially if material from different weeks or units regularly appears in the various assessments. Miller noted that “when you prioritize spacing and interleaving in your course design, you create a much more complex set of deadlines for students” (p. 110), which may lead to hardships for students who sought out the online learning environment because they needed a more flexible learning schedule. Here, as in many areas of teaching, you may not be able to distribute deadlines quite as much as the literature on interleaving would recommend. Even a small bit of attention to the distribution of deadlines and spacing of material should help, though.
For blended courses, you might think about how the class and online components can work jointly to combine blocked and interleaved learning. Perhaps in your face-to-face sessions you concentrate on specific topics or skills, blocking those into your 50- or 75-minute sessions with your students. Especially if you meet the students only once per week in person, you might find you need to use that space for blocked learning to give them enough initial mastery and confidence to tackle the online work. In those online assignments and discussion boards, in addition to directing them to the recently covered material, you could continually push students back to older material or ask them to draw connections between the material covered in a recent class and previously covered material. You can also make use of staggered deadlines to help get the most power from those online components. You can just as easily reverse this strategy, giving online assignments in ways that will focus their attention on specific skills and using your face-to-face time to require them to mix practice and pull skills and ideas from throughout the course. Neither approach seems inherently better to me; it likely depends on the type of material you are teaching. However, it seems to me like a natural fit to make deliberate use of face-to-face and online course components to support both blocked and interleaved learning, whether you are doing so in a fully blended course or even in a traditional face-to-face course that uses any of the features of a learning management system, including quizzes or discussion boards. All these recommendations represent small design shifts that can be addressed as you are laying out the basic plan for your course.
As you devise your own techniques to incorporate the power of interleaving into your courses, remember that you can use both spaced (or distributed) learning and interleaving to boost long-term retention. The smallest teaching step would be to find simple ways to space out student exposure to key course material through cumulative quizzes and exams. If you see positive results, you can then work more gradually on how to design an assessment system that creates more fully interleaved learning.
We can once again look to those fertile opening and closing minutes of the class period for interleaving techniques. But every one of your assessments, from quizzes to papers and tests and presentations, can become a potent tool in your interleaving arsenal.
Interleaving brings us to the conclusion of the Knowledge section of the book, but the benefits of interleaving are not restricted to the memorization of fact or the mastery of content. As some of the experiments described here demonstrate, interleaving improves long-term retention in all areas of learning, from retention of facts to the mastery of higher order cognitive skills. It will prove as effective in your students' memorization of key concepts in your discipline as it will in their mastery of complex skills like writing, speaking, or problem solving. This chapter serves as an excellent transition from knowledge to understanding since an interleaved approach to learning should overlay all of your course design and teaching practices.
In short, if you want students to do well on your individual quizzes and exams and papers and projects, you can teach them through massed or blocked practice.
If you want them to learn content or skills that stretch across the entire semester, and even beyond the confines of your course, interleave.