The goal of this review is to highlight the key elements needed to successfully deploy team-based learning (TBL) in any class, but especially in large enrolment classes, where smooth logistics are essential. The text is based on a lecture and workshop given at the American Physiological Society's Institute on Teaching and Learning in Madison, WI, in June 2016. After a short overview of the TBL method, its underpinning in learning theory, and a summary of current evidence for its effectiveness, we present two case studies from our own teaching practices in a new medical school. The first case study explores critical elements of design and planning for a TBL module, and the second explores best practices in classroom management. As medical educators in the fields of physiology, pediatrics, nephrology, and family medicine, we present the objective views of subject matter experts who adopted TBL as one teaching method rather than TBL experts or advocates per se. The review is aimed primarily at faculty contemplating using TBL for the first time who are interested in exploring the significant benefits and challenges of TBL.
- team-based learning
- active learning
- small group learning
- collaborative learning
for well over two decades, the literature has been accumulating evidence that active learning produces better educational outcomes than traditional lecture-based instruction (20, 26). A recent meta-analysis by Freeman et al. (6) used data from 225 studies to demonstrate that active learning in undergraduate science, technology, engineering, and math (STEM) programs produced an average of 6% increase in examination performance and that students in lecture-based courses were 1.5 times more likely to fail. Put into a medical context, these authors point out that a clinical trial with similar effect sizes to active learning would likely be stopped for benefit to ensure that all patients got the new treatment (active learning) rather than the less effective old treatment (lectures)! There are no clear answers yet to more nuanced questions such as: “How much active learning is enough?” or “What is the best kind of active learning?” However, TBL offers one solution that allows us to break with the long tradition of experts simply telling students what they should know.
Our Goals and Learning Context
The authors of this paper are medical educators from the disparate disciplines of physiology (J. D. Kibble), pediatrics (C. Bellew), nephrology (C. Bellew and A. Asmar), and family medicine (L. Barkley). We present case studies from our teaching in the preclinical phase of the MD curriculum at the University of Central Florida. In addition to TBL, we use a wide variety of teaching methods in our classrooms that include case-based learning, problem-based learning, simulations, laboratories, and lectures. We first applied TBL in 2009, when our medical school opened with a charter class of 40 students. In this review, we aim to highlight the key elements needed to develop and deploy team-based learning sessions. Our class size has grown steadily since 2009 to a current enrolment of 120 students/year, which has given us some insight to share in adjusting to using TBL in larger classes.
Brief History and Overview of TBL
TBL is a small group instructional method in which students are guided to apply conceptual knowledge through a recurring sequence of activities that involve individual work, teamwork, and immediate feedback (21, 25). Unlike other small group methods, a single subject matter expert can simultaneously facilitate many student small groups working within a large lecture hall. The TBL method was invented by Dr. Larry Michaelsen in the 1970s when enrolment in his business school course increased from 40 to 120 students. He was concerned that his students could apply concepts to real-world problems rather than simply learning about them (22). Fast-forward to today, and through some strategic grant funding and several important collaborations, TBL is now a well-documented and accepted teaching method, validated through its adoption by many disciplines and across many countries around the world (9). The health professions are among the biggest adopters of TBL, with their obvious (albeit far from unique) concern for producing graduates who can think critically and apply concepts in practice.
The student experience of a single TBL “module” is characterized by three key phases (Fig. 1):
Phase I: Work assigned to individual students before class.
Phase II: Individual (iRAT) and team readiness assurance tests (tRAT).
Phase III: Decision-based team application exercises.
In phase I, students are given learning objectives and a work assignment to be completed individually before class. In phase II, students first take an iRAT on the assigned reading material, which typically consists of 10–20 multiple-choice questions. Immediately after the iRAT, students repeat the same test, working with their team to reach consensus answers. The purpose of the RAT is to ensure that students have enough knowledge base to tackle the higher-order problems that will be presented in phase III. The use of graded iRATs provides individual accountability for students to be prepared; tRAT forces team members to engage in and refine their team process to maximize the team score. A key feature of the tRAT is immediate feedback on the answers, which is facilitated by using special scratchoff Immediate Feedback-Assessment Technique cards showing when the correct answer is found; points are lost for each incorrect attempt (22). Instructor clarification of items occurs as needed at the end of the tRAT segment. From a philosophical standpoint, phase III is the most important part of the TBL process, in which students are presented with higher-order application exercises. These are usually cases or other real-world scenarios that require teams to make a decision through application of course concepts. As discussed in more detail later, the application problems should be structured using the “4S” principle (“Significant” problem, groups work on the “Same” problem at a given time, “Specific” choice, “Simultaneous” report).
Opportunities for student appeals are a feature of the graded components of TBL and are intended to be another learning opportunity. Appeals are only allowed if a group believes there is a better answer, which requires references and a rationale, or if the question is ambiguous, which requires students to explain and rewrite the question. Appeals are carefully considered by faculty, who decide if they will be granted. This basic three-phase process represents a recurring sequence during a course, with the number of TBL modules determined by the scope and design of the overall curriculum.
The Seven Core Elements of TBL
The pioneers of classical TBL recently postulated a model in which the beneficial learning outcomes of TBL were mediated by learner engagement both with the material to be mastered and through engagement with other learners (9). Seven core design elements were identified that faculty should employ to maximize learner engagement and which should ideally be present to define a learning activity as TBL (2). The seven principles are 1) team formation, 2) readiness assurance process, 3) immediate feedback, 4) sequencing of in-class problems, 5) use of the 4S principle, 6) establishing an incentive structure, and 7) peer review. Most of these are included in the recurring steps of a TBL module described above, except for elements 1, 6, and 7, which are important nonrecurring parts of TBL discussed below.
The success of TBL depends on high-functioning student teams. In our school, team formation occurs as part of a 3-h TBL orientation at the start of the first year. Forming diverse groups requires careful thought about the “wealth characteristics” that need to be spread evenly between groups. For example, we identify students with 1) prior experience as healthcare providers (e.g., nurse, EMT. etc.), 2) higher degrees, 3) undergraduate science majors. We then ensure that each group has representation from all of these categories before checking for balanced sex and ethnicity; groups are formed in the classroom so that students can see that there are no hidden agendas. TBL teams, like other small groups, experience a fairly long development process summarized in 1965 by Tuckman (30) as “Forming, Storming, Norming and Performing”. We keep TBL groups together for 1 yr, over which time they transition from voting on every issue to eventually having inclusive discussions to reach consensus. Development of teamwork competency is a critical learning outcome for medical graduates that has been shown to be assisted through TBL (12).
In classical TBL, a proportion of the summative grade is assigned to each component: iRAT, tRAT, application exercise, and peer review. A typical score breakdown might be 20% iRAT, 35% tRAT, 35% application exercises, and 10% peer review. During orientation, students are invited to debate, within limits, how the score breakdown will work for their courses. This always produces vigorous debate and forces students to explore the relative merits of individual effort and teamwork and trust issues among peers. Many students are surprised to learn that the lowest-scoring teams usually outperform the highest-scoring individuals (21). The night before our orientation, students are also given a TBL paper to review, and they experience all parts of a TBL module during orientation to set the stage for later coursework.
The inclusion of a graded peer review process at the end of the course is another method of ensuring accountability that students come prepared and are good citizens within their team. In longer courses, we include a formative peer review process at the midpoint as a training activity and spend time discussing examples of good and bad feedback. Peer review is given anonymously, with part of the score coming from Likert scale questions and the remainder based on how well a student writes feedback about their peers. The giving and receiving of feedback is an important learning outcome for MD graduates and is one reason we value TBL in the curriculum.
Is TBL Effective?
The TBL literature is maturing and has recently yielded two systematic reviews addressing learning outcomes (5, 8). When considering knowledge acquisition, TBL is on average superior to lecture-based programs (4, 15, 16, 34). TBL seems to be particularly effective at increasing the performance of the weaker students in a class (14, 15, 16). There is also evidence of increased team skills (12) and problem solving (19) and a couple of examples where actual workplace performance was shown to improve (3, 29). A striking finding in the systematic reviews is the high proportion of published TBL articles that do not meet inclusion criteria because they do not conform to the seven core elements; a lot of “modified” TBL has been described, creating difficulty in scientifically evaluating outcomes (2). Although student learning outcomes generally improve, it is also important to point out that student satisfaction outcomes are mixed, with examples of both increased student perception scores (17, 18) and decreased student satisfaction (10, 12, 33). A more detailed study of medical student perceptions by Parmelee et al. (23) reveals considerable variation in student satisfaction with different aspects of the TBL process (e.g., peer evaluation is rated lower) and also that ratings change with time (e.g., second-year students becoming less enthusiastic about peer evaluation or the sense that TBL aids their professional development compared with first-year students).
Finally, it is reassuring to note that TBL is solidly grounded in constructivist learning theory (11). For instance, during the TBL process, learners are exposing inconsistencies in their existing mental models that force current understanding to be remodeled with new mental frameworks that can incorporate new ideas and knowledge. The application exercises in particular pose real-world problems that require critical thinking. There is a distinct shift in the role of the teacher from being the main active agent in the classroom to being an instructional designer, a facilitator of learning, and a vehicle for timely feedback. TBL makes extensive use of social context through incentivized teamwork, which further assures that it is solidly grounded in modern learning theory (13).
Case Studies from Our Institution
The first two preclinical years of our MD curriculum are illustrated in Fig. 2. Modules in the first year integrate the fundamental basic science disciplines (e.g., physiology is taught with anatomy in the structure and function module). The second year teaches pathophysiology and pharmacology using a body systems approach. Our doctoring course (Practice of Medicine) runs concurrently and is integrated with the other modules (e.g., physical examination and anatomy are coordinated). Students also conduct an independent research project across this 2-yr period. Almost all of the courses include some TBL as a teaching modality, typically with a 2–3 h TBL event every 2 wk; the Psychosocial Issues course during first year is an exception and is a novel behavioral science program taught entirely by TBL (31). In the third year (not shown) students rotate through six core clinical clerkships, and one of them (psychiatry) also uses 100% TBL for the didactic teaching component. Two of our authors provide personal narratives below from the perspective of faculty getting started with TBL. CB refers to the Renal and Gastrointestinal System course in year 2 and LB refers to the Psychosocial Issues TBL course in year 1.
We conclude with a commentary on these faculty experiences to highlight key issues when implementing TBL, especially with a large class. Our collective experiences align quite well with the five general factors for TBL implementation identified by Thompsen and colleagues (27, 28) in 2007: buy-in, expertise, resources, time, and course characteristics, which we will amplify below.
Case study no. 1, Dr. Bellew: design and planning a TBL capstone for renal pathophysiology.
I am a pediatric nephrologist whose previous position was primarily clinical, with teaching responsibilities at the clerkship and residency level. Having recently transitioned to a position that was primarily education-based with teaching responsibilities in the preclerkship/basic sciences years of an MD program, my tasks included developing and facilitating TBL sessions. At this point, I had no experience with TBL (realizing quickly that the repeated references to “RATs” had nothing to do with small furry creatures). I had to learn TBL from scratch. I was able to participate and/or observe several TBL sessions in the curriculum, review articles on the structure of TBL, and attend a 1-day workshop put on by the TBL Collaborative on the basics of TBL design and facilitation. Having completed all of this preparation, I was able to begin working on developing and revising TBL modules.
The Renal/GI module is taught by multiple faculty members using several instructional methods, with TBL being only a small part. In our growing medical school, it became quickly apparent that some active learning sessions in the large classroom that were effective with 40 students became messy with 60 students and downright chaotic with 80 students. With the final class size of 120 students quickly approaching, new sessions needed to be designed, and existing sessions needed to be revised with that in mind.
We do not work alone at our institution. A high priority is placed on integration of material across modules and between academic years as well as collaboration between faculty members. Many sessions are not developed by a single faculty member but by faculty working together. I was able to work closely with an adult nephrologist (Dr. Asmar) who had much more experience in preclinical education than I did, albeit not in using TBL.
Next, the overall goal of the TBL and its place in assessment needed to be evaluated. The renal and GI module covers three large blocks of material (renal, gastrointestinal, and hepatobiliary) that would be assessed mainly with a final multiple-choice summative exam. We decided that a TBL would be used as a capstone to each one of these blocks. We would not be covering new material but applying the material already presented. The idea was to make students accountable for keeping up with the material, to develop their clinical reasoning skills, and hopefully to prevent the tendency to simply “cram” prior to the final exam. In addition, this was an opportunity to reveal and correct misconceptions.
The timing of TBLs chosen was intended to incentivize keeping up, with the advantage of taking some of the pressure off the summative final exam but without being so frequent as to add a detrimental amount of stress to the students in the medical school environment. In addition, a decision needed to be made as to how each TBL would be graded. The assessment in traditional TBL comes from four components: iRAT, tRAT, application exercises, and a peer evaluation. Would the TBL have all of these components assessed or would it be modified based on its use in the curriculum? The first TBL to be developed and subsequently modified as the class size grew was the Renal TBL, and this is discussed further below as a model of how we planned each TBL module.
We started by thinking about the application exercises, where the main goal was to have the students think critically and apply the information that had been covered so far in the course. We also wanted to develop their clinical reasoning skills and encourage them to be able to accept a certain level of uncertainty. With this goal in mind, we developed a session that involved a progressive case structured so that the groups simultaneously reach certain key break points for class discussion.
The case began with a written history and physical exam and a few basic laboratory results of a patient presenting with an acute glomerular nephritis. We asked each group to develop a differential diagnosis with this limited information. This was a free text response where the students would write their answer on a large white board, and all groups would display their differential to the class as a whole at the same time. This would give the other groups a chance to see how the rest of the class interpreted the information, and it gave the instructors great insight into how the students were interpreting the course content. It turned out that large misconceptions that we had not seen coming became clear, and we were able to correct these for the whole class. However, as the class size approached 120 students with 20 groups, this became unwieldy. To keep this important aspect of the TBL session, we had the student groups come together in pairs to agree on what differential they would display to the class as a whole. This gave the individual groups a chance to defend their thought process to another set of students and reduced the number of differentials displayed to a manageable number (20 became 10). This process was very insightful into the students' thought process but took a large amount of time. The remainder of the TBL needed to be time efficient while still stimulating critical thinking and discussion.
The next step in the case was to choose how the patient would be worked up further. This problem would be posed in the form of a multiple-choice question where each answer was a group of laboratory and/or imaging studies. However, we didn't just want the students to be able to narrow it down to a single correct answer through a process of elimination. We wanted to force them to make decisions with imperfect information where there may not be a perfect answer and be able to defend their choice. As a result, we chose to not have an answer that included a complete workup. The students would need to understand what type of information was going to be obtained by each of the laboratory studies listed and how that information would help them narrow down the diagnosis. This would hopefully guide the conversation into the mechanisms of the different disease processes and how they would alter measurable parameters. They would also need to be able to justify why they would be able to live without the information not included in their chosen answer. This would require a judgment call. The different groups were likely to make different judgment calls and, therefore, pick different answers. Students describing their reasoning to defend their choice would again lead to a rich discussion with the class as a whole.
The consequences of this is that although there was going to be one answer that really was better than the rest (and, therefore, the correct answer), it would not be purely black and white. If this was a graded question, there would be a danger of having the discussion focus on “getting the point” and becoming contentious rather than thought provoking. To eliminate this issue, we decided not to grade the application exercises.
At this point, the main objectives of the TBL were met. However, we wanted to wrap up the case for the students so that a few more quick questions were added. The students would receive the results of the actual patient workup as well as pathology images from a biopsy. The students would get the chance to finally make the diagnosis and get some brief information on the treatment of the patient to bring closure.
Since our purpose was to apply already covered material, iRAT/tRAT questions were developed to determine whether the students had a broad understanding of the main concepts covered in the renal block necessary for tackling the complex case problem. The preparation materials would not include any new content, but the students would be instructed to review learning materials from the preceding 2 wk.
Case study no. 2, Dr. Barkley: sensitive topics in behavioral medicine pose a challenge for classroom management.
My experience with TBL started in 2012 when I volunteered to direct our Psychosocial Issues in Medicine module for first-year medical students. The 12-wk module was taught entirely with TBL, and I had never heard of TBL before this class. I also am a physician and my teaching has been mostly with medical students, residents, and fellows in clinical settings, so I was new to large classroom teaching as well as the TBL methodology. I did know that the students had some strong opinions about the class and a love-hate relationship with the TBL method. I read about the method, attended a Team Based Learning Collaborative meeting, and got input from other faculty prior to the course.
One of the hardest adjustments for me was the different role that the faculty member has in the TBL method. I was used to lecturing and felt most comfortable with that method. It was hard for me to feel prepared for class, as I would not know what aspect of the material that the students would have questions about. This made it hard to know which details to clarify in my own mind. The first year, I participated in all 12 sessions in the module, and for some of the topics I did not have a deep depth of knowledge. I have learned from this that it is better to have faculty participate in the session topics in which they do have a deep level of knowledge and experience. Since the discussion and questions from students can go in multiple directions, if the faculty member has a breadth of experience in the topic, it will be easier to feel comfortable with the discussion. This experience with the material issue is especially important in the class that I teach, which has to do to physician/patient communication and the impact of social factors on health. This material is not black and white and there are nuances in how to manage the clinically related topics. Therefore, it is important to let the students know how faculty would handle certain psychosocial situations in practice. It has been helpful for me to take notes after my classes on the questions that the students had a hard time understanding and use this information to add more specifics on concepts to focus on in the reading guide for the week or to change preclass reading materials.
Another lesson I learned was how to strike a balance between adding my clinical experience and knowledge to the discussions and letting the students discuss the material from their perspective. At first, I didn't add as much of my input as I wanted the students to discuss. At times, this led to frustration in the class, as they did not really understand the faculty reasoning for the right or wrong answer, and more explanation was needed to get this across. I have found that both faculty and student input are essential, and timing in the discussion on when to give your input as faculty is very important in gaining the most learning from the discussion sessions.
Managing the class to facilitate discussion is another very unique skill that is needed for TBL implementation. This is especially important in large class TBL facilitation. When our class size got to 100, I noticed that facilitation became more challenging. We had to consider whether everyone could hear the discussion and started using a microphone that had to be passed around the room. Also, it was harder to engage multiple students, and we had to have the teams rotate who would respond for the team.
Finally, disagreement between the faculty and students over the correct response was something that I did not anticipate would be a consideration in professional school education. This area demonstrates for me how powerful TBL can be in facilitating student growth as professionals. There were times when the students did not agree with the faculty rationale for the correct response, and the class discussions went beyond trying to understand the right answer to just wanting to get the points. This meant that faculty had to model professional behavior for managing disagreements and at times bring students back after class to discuss how their behavior could be changed. It is extremely important in these circumstances for the academic administration to support faculty in managing the expectations for professional behavior. It is also important for students to feel that they are heard and to make grade adjustments when they have an alternative view of the right response that can be supported by the assigned reading material. The appeals process helps with this aspect of class management, as faculty can end a discussion by asking students to submit an appeal and discuss their concerns after the class session.
Overall, my experience with TBL has been very rewarding. It really helps me in planning learning experiences overall, as I am more aware of how to incorporate methods that will facilitate student discussion, reflection, and higher-order thinking skills. I am also more comfortable in balancing student perspectives and my professional experiences in discussions. These outcomes could not be achieved in a large class setting with the traditional lecture format.
Before dissecting these real faculty experiences, it is striking how authentic the learning experience is for something happening in a lecture hall. Drs. Bellew and Asmar present students with a detailed, realistic workup of a patient and make the students accountable for obtaining the right medical tests and diagnosing the patient's problem. Dr. Barkley talks about how she brings in the context of her medical practice during discussion, of presenting students with areas of uncertainty in real-world decision making, and of sometimes provoking emotional responses in students that lead to discussions about professionalism. Dr. Barkley's topics are challenging, such as sexuality, death, domestic violence, addiction, and health disparities in society; surely such topics should evoke strong emotion in trainees rather than be reduced to a list of facts in a lecture!
A key factor both stories bring out is time investment by faculty. Drs. Bellew and Barkley are both clinicians who started by investing significant time to attend training workshops and had the courage to step outside their comfort zones. The initial time investment in developing or preparing to facilitate TBLs for the first time is evident. This shows that these faculty had bought in to the idea that TBL could help their students go beyond knowledge acquisition. Buy-in is also needed by administration. In our case, resources for faculty development were provided as well as permission to have a significant TBL footprint in the MD curriculum (see Table 1). Dr. Barkley also notes that administration will need to be supportive of faculty efforts to incorporate active learning when navigating instances of student discontent. Most students buy in to the benefits of TBL, but some resist the higher level of accountability that comes with preclass preparation and a discussion-based classroom. Student resistance is more likely when there are concurrent courses or sessions that use traditional lecture-based pedagogy. The narratives show how challenges were placed in front of students in terms of being accountable for clinical reasoning, working in teams and developing professionalism, and not all students are likely to appreciate you for this!
As Dr. Bellew describes the development of the renal capstone TBL, note how she and Dr. Asmar are employing a backwards design approach (22) by first considering the curricular context and then focusing on designing applications exercises to meet their learning outcomes of application of clinical reasoning. As an observer of these sessions and a faculty member who once tried to follow this 3-h event with an introduction lecture to the next block, Dr. Kibble noted how truly engaged students were, how they were treated like colleagues and expected to justify every decision they made about the patient, and how exhausted they were at the end of a fulfilling TBL session!
Another critical design component Dr. Bellew has described is the use of the 4S principle (22, 25); the “Significant” problem is a patient with renal failure, groups are working on the “Same” problem at a given time, they are making “Specific” choices (listing the top 5 differential diagnoses or displaying an answer to a multiple choice item), and they are making “Simultaneous” reports to the class. Sticking to this 4S design is very important for a successful class in our experience.
Dr. Barkley brings up the topic of needing expertise in the topics. We have an integrated curriculum in which several topics or disciplines are often present in a single TBL session. Unlike problem-based learning where the non-expert tutor can facilitate information gathering by trainees, TBL needs a subject matter expert in the room. We have learned to have small teams of 2–3 faculty present when the material demands it, to allow closure on contentious topics before a classroom session ends, or to enrich debate on difficult areas. Note how CB and AA included some final shorter items to bring the patient's story to closure with correct diagnosis and treatment.
Dr. Barkley alludes to resources, especially as the class size grows. We have a lecture hall where 20+ small groups can sit on the same eye level to discuss; we use radio microphones for students to speak to the whole class, and we set expectations about respecting the person speaking. Dr. Asmar introduced an effective intermediate step of having two groups debate their answers prior to bringing the whole classroom together as the class size gets larger. Room resources, logistics, and classroom etiquette definitely need to be considered when doing TBL in larger classes.
A critical aspect of successful TBL, which becomes more acute in larger classes, is facilitation of discussion and management of the classroom. Dr. Barkley talks about needing to decide when and how to bring the faculty perspective into the student discussions between groups and the need to avoid student frustration by ensuring that faculty bring clarity to why particular answers are right or wrong. Gullo et al. (7) provide their 12 tips on TBL facilitation and also point out the importance of bringing closure to discussion and addressing lingering disagreements. We have found these to be critical in avoiding student discontent with TBL. There is a careful balance needed to first bring out all the different student points of view by restating points and posing neutral and open-ended questions (“Why did your group think that?”, “Did your group have a different way of approaching the problem?”, “Can you expand on that?”, etc.). If the room is quiet, it is important to wait for students to answer and to set an expectation that every individual is accountable and may be called upon to answer. After student discussion is finished, or unresolved, the faculty expert can offer context and opinion and should provide concise explanation “The faculty felt the best answer was X because . . .” However, you must not lapse into a lecture at this point! One difficult aspect is keeping a close eye on the time. It is easy to run out of time, so faculty need a plan beforehand for roughly how long can be spent on a given part of the TBL.
Dr. Barkley talks about student dissent, and some faculty find it hard to cope with this in the classroom. Students need to be aware that we welcome and expect disagreement and even passionate exchanges but also that we operate within professional boundaries. Universities are supposed to be places where it is safe to disagree and debate. A common problem in this environment is that faculty put down any dissent with an authoritarian response and at the same time kill future discussion. We find that it is better to explain the faculty viewpoint, acknowledge the student discord, and invoke the appeals process; appeals are invaluable as a means to have everyone pause, reflect, and properly articulate their disagreement, and they allow the class to move forward. The professionalism curriculum is also supported by including a peer review process.
In conclusion, we find that TBL can be a very effective way to support learning outcomes that go beyond knowledge acquisition and even reach into difficult areas such as the professionalism curriculum. Faculty need to be aware that they are giving up some control over events in the classroom, and they need to be comfortable working in a noisy, high-energy, and less predictable environment. We strongly recommend taking some faculty development sessions, following the recommended planning elements for TBL and getting your first modules reviewed by colleagues with experience in TBL.
No conflicts of interest, financial or otherwise, are declared by the authors.
J.D.K., C.B., A.A., and L.B. conception and design of research; J.D.K. analyzed data; J.D.K. prepared figures; J.D.K., C.B., and L.B. drafted manuscript; J.D.K., C.B., A.A., and L.B. edited and revised manuscript; J.D.K., C.B., A.A., and L.B. approved final version of manuscript.
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