Unit Overview

A hypothesis, or scientific claim, is simply a well reasoned guess at the answer to your research question.  The words “well reasoned” are important here.  A well reasoned claim usually includes a 'because' statement or takes the form of an 'if, then' statement. For students to develop a such a claim it should grow out of some kind of a model of the system or mechanism under investigation.  A student ought to have some idea as to why the outcome predicted by the hypothesis would happen.

Example: Suppose a student offers a scientific claim that mosquitoes would have higher mercury levels than beetles.  Then suppose you ask the student to explain why he or she expects this outcome.  If the student simply says, “I don’t know, it’s just a guess,” then there is no model in place, and when the student actually gets the data back from the lab all that he or she will be able to say is either “Good, I was right,” or “Huh, I was wrong.” 

But if the student has a model of how things work that is informing that scientific claim—saying, for example, that since mosquitoes get their food from animals, some of which are carnivores, they are effectively high on the food chain—then there is the possibility of having some learning happen.  If the data support the hypothesis, you might suggest that the student compare the mercury level of male and female mosquitoes to see if the blood is what makes the difference.  If the data do not support the hypothesis (often a more interesting outcome), you might suggest that the student try looking exclusively at female mosquitoes, or compare the mercury level in mosquitoes to that found in other insects, and so on.  The point is that the model behind the hypothesis gives you something to build on.

Coming Up with a Hypothesis

We have found that students have difficulty coming up with a hypothesis that they can work with.  Perhaps some of the problem is that “hypothesis” is a formal-sounding word.  An approach that has worked has been to have the students review their learning so far and then ask “I wonder” and then “I think.” And have students do this often. Hypotheses often come from a continuous refinement of ideas.

Have the students start with the question that the class is looking at. Suppose, for example, that the question is, “How does mercury end up in living things in a forest?”

We suspect that a lot of the trouble that students have is in moving from this kind of general question to the more specific kind of assertion that makes for a hypothesis.  Just as in our discussion of “well reasoned guess” above, what is missing is the model.

So, you would probably want to have the students draw a picture of how they think mercury moves in the forest (see The Mercury in Foodwebs classroom activity). Where does it come from and how does it enter the food chain? Different students will come up with different pictures, or models, of course—but suppose that a group of students has the mercury captured by the trees, washing to the ground, moving into its methyl form in a boggy area, getting absorbed by plants in the bog, moving up into insects that feed on vegetation in the bog, and so on. 

That model is what generates some hypotheses.  With a model, the students can ask, “If the model is correct, what would we expect to see?”  Would the algae and plants in the boggy area contain more mercury than other plants?  What kinds of insects or other animals feed in the boggy area?  Would they have higher mercury levels than upland insects?  And so on.

The hypotheses grow out of following out the implications of the model.  Once again, “If the model is correct, what would we expect to see?”

So, how do the students go about constructing a model?  They already made a start at this at the end of Unit 2, when they drew diagrams of the mercury cycle for whatever system it is that they are studying.  Some of the information required to understand and extend this diagram comes directly from the course content that you provide, using the resources in these lesson plans, the class text, and so on.  Another part of what the students need will come from additional reading about the plants, amphibians, and other forms of life in the system that they are studying.  The key thing is to have the students try to draw a picture of how the system works.  When there is a part of that picture that is fuzzy or that they don’t understand, they need to do some more reading, or—devise an experiment to find out what they need to know.

Note, by the way, that the model that the students come up with does not have to be TRUE in some final sense.  It just needs to hold together, make sense, and serve as a good foundation for their continued thinking and research. The degree to which you want to interfere and “correct” a model is another one of those judgment calls that depends, once again, on whether the model is:

• owned by the students
• on the right topic
• pedagogically useful

A Hypothetical Chart

It would be a good idea to have the students make a chart or graph of what they think the results of their data-gathering might be. This would give them an indication of the viability of their hypotheses. They should draw the graph on a piece of paper rather than use a computer. They will need to decide what information goes on which axis and plot what they think the data will show. For example, say a student hypothesized that mosquitoes will have more mercury than beetles because mosquitoes feed on carnivores which are higher on the food chain. Here is what their hypothetical chart would look like:

A Good Resource

We have found that a simple "Claim, Reasoning and Hypothetical Graph" worksheet coupled with classroom discussion and peer review help with developing scientific claims.

Goals & Challenges

The goal is to guide the students toward a specific research question—a question that they can investigate by setting up an experiment, collecting data, and analyzing the results. Teachers need to guide this process so that the students’ questions meet a few important criteria:

• Owned by the Students:  Students should develop their own questions because they will learn from doing so and will be more invested in the research outcomes.
• On the Right Topic:  On the other hand, the students’ questions need to be consistent with the learning objectives for the course.  For example, if the course is focused on food webs, you might not want to let a student pursue a question about whether you catch more bass in deep water than in shallow water—unless the student connects that question to food webs.
• Pedagogically Useful:  You will also want to be sure that the questions that students are pursuing are ones that they can hope to investigate given the time and equipment that is available and given the students’ level of expertise.  You are trying to direct the students toward a positive outcome, and you want to steer them away from paths that are likely to end in frustration or in a trivial, uninteresting result.

Balancing these objectives requires judgment, familiarity with the scientific principles involved, and practice.  You, the teacher, are acting as a guide for the students on an intellectual adventure.  Some journeys inevitably turn out to be more interesting and exciting than others.  The more times that you lead these trips, the more you will know about what works.  This is one of the key reasons that we are asking you to share your experiences, both good and bad, with other teachers.  By pooling experience you can learn from each other.

Key Ideas introduced in this Unit:

• A hypothesis grows out of some type of model
• Models are generated through a series of related questions
• Hypotheses are neither right nor wrong; they are either supported or not supported
• Questions can be answered by setting up an experiment, collecting data and analyzing results

Concepts and Relationship:

Students should:

• Have an idea of which organisms would contain greater amounts of mercury
• Have an idea of where organisms are on the food chain (who eats whom)
• Have a good idea of the mercury cycle
• Be able to relate their study site to more abstract concepts that drive mercury bioaccumulation and patterns (for example, think about whether their site has a lot of wetlands and recall that scientists think wetlands are ‘hotspots’ for mercury methylation and transport).

Vocabulary:

• Hypothesis—an educated, well reasoned guess at the answer to a research question
• Claim—an assertion of something
• Evidence—the medium of proof
• Reasoning—the drawing of inferences or conclusions

WORDS TO AVOID: Fact, prove, true, right, wrong, good, bad

We want to avoid using words like those above because they reinforce the notion that science and the interpretation of data are absolute, or black and white. Really, we are gathering evidence that supports or does not support a hypothesis. We can never know everything about every part of a system, so there’s always a chance we’ll find out something new down the road that will change how we think the system works. For example: Newton thought that light was made up of particles – for good, logical reasons. Later, when scientists observed diffraction, they started to wonder if it wasn’t a wave instead. We now think it travels as both particles and waves. Newton wasn’t wrong. He was basing his hypothesis on sound evidence, but as new evidence came to light our understanding shifted somewhat. It may shift again as we record new observations and phenomena regarding the properties of light.

Student Prerequisites:

• How to measure volume and square inches
• How to convert units such as µg/g to ppb or ppm (see Introduction to Mercury in the Environment Essential Activity- Why do we care?)
• Proportions
• Averages
• Ranges
• Computer skills
• Library skills

Misconceptions

• Hypotheses are difficult to develop
• If the data don’t support my hypothesis, I got it wrong (or if the data do support my hypothesis, I got it right)