Problem-based Learning in Science

Ill-Defined Encounters Are the Right Kind!

(guiding problem-based learning in science classrooms)


The best way for students to learn science is to experience problems that challenge science, and the thought, habits of mind and actions associated with trying to solve them. This implies opportunities for authentic, inquiry-based learning. Problem-based learning (PBL) is a powerful vehicle for this, in which a real-world problem becomes a context for students to investigate, in depth, what they need to know and want to know (Checkly, 1997). It is a robust, constructivist process, shaped and directed primarily by the student, with the instructor as metacognitive coach.

PBL is not just another iteration of what many science educators already use in their classrooms. To be truly "problem-based", Gallagher (1995) emphasizes, all three of these key features must be present: initiating learning with a problem, exclusive use of ill-defined problems and teacher as metacognitive coach.


The nature of ill-defined problems

At the heart of true PBL is an ill-defined problem, an unresolved "murky" situation. This is presented to small groups of students who have been given a stakeholder role which is the "hook", says Gallagher (1995), that propels and invest students in the ill-defined situation.

To better understand what is meant by an ill-defined problem it is helpful to examine what is meant by a problem. Although problems can differ in many ways, they all can be considered as having three characteristics. First, there is an initial or present state in which we begin. Second, there is a goal state we wish to achieve. Finally, there is some set of actions or operations needed to get from the initial state to the goal state.

While all problems have these components, they often differ in how well-defined they are. Problems can vary on a continuum from relatively well-defined to ill-defined along each of these components. In PBL, the problem is ill-defined with respect to all three characteristics, which is typically how problems present in science (and life!). The "problem" is unclear and raises questions about what is known, needs to be known and how to find out. This opens the way for finding many problem possibilities, the nature of which are influenced by one's vantage point and experience.

In typical classroom problem solving approaches, students encounter problems after all information is taught, giving the misleading impression that problems only arise in circumstances where all information needed for solution building is available. In PBL, Gallagher (1995) emphasizes, the order of learning is inverted to reflect real life learning and problem solving. Learning begins after students are confronted with an ill-defined problem.


Science, learning, and problem-based learning

The theme of science education reform is to understand science as ways of thinking and doing as well as bodies of knowledge. Emphases are thinking and problem solving and habits of mind that promote exploration and discovery such as curiosity, questioning, openness to ideas, learning from errors and persistence. Learning needs to occur in the context of real investigation through inquiry and reasoning, which means teaching for understanding not memorization of facts (AAAS, 1989; NSTA, 1992).

Learning specialists concur. Wiggins and McTighe (1998) advise that learning is best, much more takes place, when the learner is the one who looks deeper to create meaning and develop understanding. Understanding, Perkins and Blythe (1994) explain, is deep learning that goes well beyond simply "knowing", such as being able to do thought-demanding things with a topic like finding evidence and interpreting information in new ways. Wiggins and McTighe (1998) stress that students need to "uncover" content for meaning, to question and verify ideas if they are to be understood, and Caine and Caine (1997) emphasize that the mind needs to be understood as purposive, self-reflective, creative, and requiring freedom to create meaning. For these reasons, advise Wiggins and McTighe (1998), a priority in teaching for understanding is shaping content in ways that engage students in making sense out of it through inquiry and application.

In PBL there is a shift in roles for students and teachers. The student, not the teacher, takes primary responsibility for what is learned and how. The teacher is "guide on the side" or metacognitive coach in contrast to "sage on the stage", raising questions that challenge students' thinking and help shape self-directed learning so that the search for meaning becomes a personal construction of the learner. Understanding occurs through collaborative self-directed, authentic learning, characterized by problem-finding, problem solving, reiteration and self-evaluation. This, says Barrows (1997), is what distinguishes true PBL from "same-name" methods that use a problem of any sort somewhere in the teaching/learning sequence.

In PBL, Gallagher (1995) explains, students encounter a problem as it occurs in the real world, outside the classroom. There is insufficient information to develop a solution, no single right answer or strategy, and a need to redefine the problem as new information is gathered. Ultimately, students can't be sure of their solutions because information will still be missing. This also characterizes science, which one scientist I interviewed describes as "a process of thinking about problems then designing means of approaching them... not necessarily to solve the problem you outlined, but to make an inroad or a start, asking what further approaches can I use to get a handle on this problem?"


Connecting students with scientists

An exciting way to launch students into the process of science is to link them with practicing scientists and their work. This led to my interviewing six prominent biomedical scientists, during which each was asked to describe a difficult, especially challenging research problem. They were also asked to discuss their concept of science and important thinking behaviors for scientists. Students need to learn, first hand, about this "private side" of science, the essential habits of mind and thought processes that promote exploration and discovery.

These conversations by four men and two women, physicians and Ph.D.s, are intended as catalysts for students to conduct their own interest-based inquiries through a model I developed for problem-based learning. Each scientist discusses perplexing aspects of their particular research which may be on cancer, organ transplantation, heart disease, AIDS, the treatment of wounds and burns, substance abuse, or human response to environmental toxins. Embedded in these talks are many possible problems students can unearth, then choose from to investigate using the "Steps in PBL" model. The model follows, with examples of thinking by a group of high school biology students who applied it to one of the conversations.


Guiding students in PBL

This ten-step approach (Figure 1) is based on the original medical school model (Barrows, 1986). It involves students in constructing understanding through critical and creative thinking and promotes collaboration and autonomy in learning:

  1. Encounter an ill-defined problem: Students can encounter real-life, ill-defined problems in many compelling contexts. As stakeholders in a situation they might be environmentalists investigating a pollution problem or scientists confronting a puzzling research finding. In the following scenario, biology students are a special interest group attending the presentation, "Programmed to Die" (text from one of the scientist interviews):

    Picture ID's identify you as invited guests to a cancer research presentation in the hospital conference room. Your organization, Science in the Public Interest , does more than report medical research findings to the public. It questions them and actively explores further meaning for public consideration. You listen carefully as "Programmed to Die" begins...

    Metastasis... cells dividing out of control. That's what kills us. Finding a cure for cancer is a difficult problem because all cancers aren't alike. I'll make the analogy to infectious diseases like HIV and influenza. Their treatment and how they cause disease differs. Colon cancer behaves different than melanoma, which behaves different than prostate cancer. While some things are similar, like cells dividing out of control, they behave differently. Melanoma likes to go from the skin to the brain and liver, not to bone or lung. Colon cancer goes to the liver, breast cancer to lung, bone and brain. It's like these cancers have zip codes. There's selectivity as to where they go and set up shop. So it's been a cure versus cures. There's a 95% cure rate for testicular cancers. Hodgkins disease once had a bad prognosis, now 90% survive. But this doesn't apply to breast cancer or other important diseases.

    We've been studying metastasis by looking at genes that are expressed in a tumor cell versus its metastatic components - to understand the molecular differences between the original tumor and one that went to the liver. En route, we discovered a new gene that keeps a melanocyte in its normal state and tends to prevent its progression to melanoma. As melanoma develops, this gene is no longer "expressed". In science, you often pursue directions that are different from your primary focus and wind up discovering things that might be related, like a gene that's important in the fruit fly or worm that's also found in humans. In the worm, we discovered something we call "programmed cell death", which is an exploding area of cancer research.

    In the development of the worm there are certain cells. When cells divide in twos, one gets discarded and dies. It's meant to die. If not, it's a problem. If a cell's DNA is damaged and isn't repaired it's supposed to die. If not, it leads to cancer. Not all cells produced are meant to continue to be produced. Normally, you divide the two. Each one should have a function. We did experiments to interfere with that and the animal's whole system became abnormal. If cell death is important in a worm, what about in humans when cells that should die don't? Normally, when a cell is damaged, there's a mechanism for repairing it. or getting rid of it. A cell dying by this mechanism looks different from one dying from other causes.

    A connection between AIDS and cancer is the immune system, "the national guard that protects our shores". Cancer cells are probably being produced all the time, but there's a lot to suggest that the appropriate immune cells get rid of them. When the immune system is suppressed and the national guard troops have gone from thousands to ten, invaders come ashore and set up shop. There aren't the natural killer cells and macrophages that get rid of cancer cells, resulting in malignant tumors that metastasize. The immune system is important but not the whole story in understanding metastasis.

    In the future, we'll be better able to determine the behavior of tumors through molecular testing. A cancer cell grows out of control. Through gene therapy, which is being tried now, we'll try to put those controls back in by reintroducing genes that either were lost or non-functional to regain behavior. Meanwhile we're searching for other solutions.

  2. Ask IPF questions: As stakeholders, biology students begin to examine this information by asking, "What's Interesting here?" "What's Puzzling, curious, problematic?" "What's important to Find out?" (Figure 2)

  3. Pursue problem-finding: Embedded in "Programmed to Die" are many problem possibilities students can unearth by probing the information more deeply for meaning, which IPF questioning initiates. To promote this, teachers can suggest varied problem finding strategies, for example:
    • draw a problem; even crude drawings can convey a lot of information
    • ask a series of "why" questions to reveal possible causes of something
    • create a flow map to sequentially link aspects of a situation
    • uncover possible false assumptions about information
    • minify or magnify a situation to understand its essence or scope

  4. Map problem finding; prioritize a problem: Next, students organize problem finding results to show patterns and relationships among ideas. Again, teachers guide but do not make decisions for students. This process needs to be a construct of the learner as illustrated by the cluster map (Figure 3) created by the biology group. Their map helps them identify "lifestyle factors and cancer" as a problem to investigate.

  5. Investigate the problem: To help the group strategize, teachers might ask: "How will you organize your overall plan?" "What responsibilities will each group member have?" Inquiry guiding questions might be, "Since you have decided to interview people, who will you interview?" "How will you find them?" "What information is needed?" "How will you record this?"

  6. Analyze results: Responsibility for analyzing information again lies with students. Guiding questions for the biology group might include: "Would it be useful to compare people you interview for similarities or differences?" "How would you show this?" "What's more important to find out: how people are similar or how they differ?" In the process, teachers might also introduce students to basic data analysis methods.

  7. Reiterate learning: Reiteration is a distinguishing feature of PBL in which students present what they have learned to each other (Barrows, 1997). They actively apply learning back to the problem to gain new understanding by re-entering it from the beginning, critiquing and refining their original problem statement, thinking strategies, sources and goals. They relate what they learned to understanding other problems and try to extract concepts that have broad applicability. Metacognitive guiding questions might be, "How do your results help you understand the problem you investigated? "Should you investigate this again, what would you do differently and why?"

  8. Generate solutions and recommendations: Students need to revisit outcomes of the previous two steps to determine what direction they take. For example, biology students' data might point to prevention/intervention. Teachers can suggest idea-generating strategies such as:
    • ask "how?" each time a solution is proposed to clarify possible strategies and implementation steps
    • propose improvements by substituting, combining, adapting or modifying ideas (Eberle, 1971)
    • use a metaphor to highlight aspects of something that might not ordinarily be perceived

  9. Communicate the Results: As stakeholders in a real-world situation, students need to communicate what they have learned. For example, biology students consider creating a public information message emphasizing the relationship between certain lifestyle factors and cancer. Guiding questions might be: "What general themes were discovered in your research?" "What conclusions can be reached?" "Who gains from this and how?

  10. Conduct self-assessment: Assessing one's performance progress is an important life skill that PBL develops. Students assess their own problem finding, problem solving, knowledge acquisition, self-directed and collaborative learning skills and share this with their group. Authentic assessment methods include journal writing, lab notebooks, self-rating scales, peer interviews, and conferences with teachers for which students develop discussion criteria. Teachers also provide their own assessments based on students' application of the 10 step model.


Encounters of the right kind

In science, questions answered lead to more questions. Understanding occurs in fits and starts, characterized by derailments, blind alleys and shifts in focus. Problems change as they are being solved, resulting in constant changing relationships between problems and solutions.

From the outset, PBL engages students in these important learning experiences. As illustrated, scientists' conversations about the challenges of their research is grist for launching students into pursuits of their own that replicate the process of science. Within the larger curriculum, this can be the basis for structuring a major piece of learning agenda over an extended period of time, or for special study to enhance a part of curriculum.

PBL gives students opportunities to be self-directed while maintaining cohesion in the classroom. It is effective with students of varying abilities because students are the ones who choose the problems and methods of study based on development level and interests. Above all, Gallagher (1995) emphasizes, PBL is a curricular and instructional approach which successfully resolves the seemingly contradictory demands of science education reform in a way that is true to the discipline of science, its process, and the larger goals of educating an independent reasoning citizenry.


© Nina Greenwald, August 2001

Greenwald, N. (2000) Learning from problems. The Science Teacher. 67 (4): 28-32

Reprinted with permission from NSTA Publications, April 2000, from The Science Teacher: NSTA, 1840 Wilson Blvd. Arlington, Virginia 22201



AAAS. (1989). Project 2061: Science for All Americans and Benchmarks for Science Literacy, N.Y: Oxford University Press.

Barrows, H.S. (1997). Problem-based learning is more than just learning based around problems. The Problem Log, (2)2, 4-5.

Barrows, H.S. (1986. A taxonomy of problem-based learning methods. Medical Education 20, 481-486.

Caine, R.M. & Caine, G. (1997). Education on the edge of possibility. Alexandria, VA: Association for Supervision and Curriculum Development.

Checkly, K. (1997). Problem-based learning. ASCD Curriculum Update, summer, 3.

Eberle, B. (1971). Scamper. Buffalo, N.Y: DOK.

Gallagher, S. Stephien, W.J., Sher, B.T., & Workman, D. (1995). Implementing problem-based learning in science classrooms. School Science and Mathematics, 95(3), 136-146.

National Science Teachers Association. (1992). Scope, sequence and coordination of secondary school science. Volume 1, The content core: A guide for curriculum designers. Washington, DC.

Perkins, D. & Blythe, T. (1994). Putting understanding up front. Educational Leadership, 51(5), 5-6.

Wiggins, G. & and McTighe, J. (1998). Understanding by design. Alexandria, VA: Association for Supervision and Curriculum Development.