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us to reject a hypothesis, or it can support the hypoth-
esis, but it cannot prove that a hypothesis is correct. To
move forward, then, we might undertake a second set of
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observations. Does pollen that adheres to the humming-
bird rub off when the bird visits a second flower of the
same species? If so, we have stronger evidence and support
for our hypothesis.
We might also use observations to test a more general
hypothesis about birds and flowers. Does the color red gen-
erally attract birds and thereby facilitate pollination in a wide
range of flowers? To answer this question, we might catalog
the pollination of many red flowers and ask whether they
are pollinated mainly by birds. Or we might go the oppo-
FIGURE 0.6 Observation
site direction and catalog the flowers visited by many differ-
Observing a hummingbird visiting a flower may lead to a number of ent birds—are they more likely to be red than chance alone
questions that a scientist can explore. Photo: Charles J. Smith
might predict?
the hummingbird—that the hummingbird’s actions reflect
its need to take in food. Both hypotheses about the hum- Designing Controlled Experiments
mingbird’s behavior provide a reasonable explanation of Our hummingbird example used observation to test
our observations, but they may or may not be correct. The several different hypotheses, but scientists often test
predictions of the hypothesis lead to further observation or hypotheses through experimentation. One of the most
experimentation. powerful types of experiment is a controlled experiment. In
Because hypotheses make predictions, we can test them. a controlled experiment, the researcher sets up at least
That is, we can devise an experiment to test whether the two groups to be tested; the conditions and setup of the
predictions made by the hypothesis are supported by the groups are identical, except the researcher deliberately
evidence, or we can go into the field to make further obser- introduces a single change, or variable, in one group to
vations predicted by the hypothesis. see its effect.
Scientists collect data by observation or experimentation, Suppose we want to understand the relationship
or both. They will then analyze the data and determine if between caffeine consumption and the heart rate when a
the data support the hypothesis. If the data do not support person is not exercising, which is known as resting heart
the hypothesis, the hypothesis is rejected. In this case, the rate. We might hypothesize that caffeine causes an increase
researcher may generate a new hypothesis. If the data sup- in resting heart rate. This hypothesis could be based on our
port the hypothesis, it gains support and further questions own experience and observations, or perhaps the scientific
can be asked. These questions may refine or extend the literature. To test this hypothesis, we can carry out a con-
hypothesis. trolled experiment. In this case, we might have two groups
Returning to the hummingbird and flower, we can test of people who are similar in terms of age, gender, socio-
the hypothesis that the bird is transporting pollen from one economic background, health, and so on. One group is
flowering plant to the next, enabling the plant to reproduce. given caffeine, perhaps in the form of a cup of coffee. This
Observation provides one type of test: if we catch and exam- group is called the test group or experimental group
ine the bird just after it visits a flower, do we find pollen because this group experiences the variable—it receives
stuck to its beak or feathers? If so, our hypothesis is sup- caffeine. In a second group, the people are not given any
ported by the data. caffeine. This group is called the control group because it
The test, however, cannot prove the hypothesis. Pollen is not exposed to the variable.
might be stuck on the bird for a different reason—perhaps Why is it necessary to include a control group? Imagine
it provides food for the hummingbird. However, if the for a moment that there was just a test group and no con-
birds didn’t carry pollen from flower to flower, we would trol group, and the resting heart rate of the people in the
reject the hypothesis that they facilitate pollination. In test group went up after they drank a cup of coffee. In this
other words, a single observation or experiment can lead case, you might conclude that the hypothesis is supported

MODULE 0 IntroductIon 7

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