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and caffeine increased heart rate. But how do you know
that the heart rate of people in the group didn’t increase on
its own? How do you know the variable that was changed
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caused the observed effect? The control group shows what
happens without caffeine so researchers can compare
what happens with and without the variable, holding every
other factor the same. In this way, they can determine if the
variable accounts for any changes in heart rate.
In a test of this hypothesis, the action of consum-
ing caffeine is known as the independent variable ,

the variable that is manipulated to test the hypothesis.
This variable is considered “independent” because the
researchers can manipulate it as they wish. The result
of the experiment—resting heart rate—is known as

the dependent variable . This variable is considered
“dependent” because it is expected to vary based on the FIGURE 0.7 Daffodils
independent variable. Daffodils, like the ones shown here, were the subject of scientific
Scientists use controlled experiments because they inquiry by James Kirkham Ramsbottom, who tried to track down
are extremely powerful. By changing just one indepen- what was causing their deaths in the early 1900s. Photo: Victoria Ambrosi/
dent variable at a time, the researcher is able to determine EyeEm/Getty Images
whether that variable is important. If many independent
variables were changed at once, it would be difficult, if the researcher accepts the hypothesis and then subjects it
not impossible, to draw conclusions from the experiment to more scrutiny by making further observations and doing
because the researcher would not be able to determine additional experiments. As we saw in the hummingbird
which variable caused the outcome. example, a hypothesis may be supported, but it is never
Our experiment testing the relationship between caf- proven because we can never know for certain whether it is
feine consumption and resting heart rate is very simple. In true in all cases.
reality, we might include more than two groups of people. Let’s now turn our attention to a real-world exper-
For example, we could test how the amount of caffeine iment that demonstrates some of the features we have
affects resting heart rate by giving several groups different been discussing. “Practicing Science 0.1: Using obser-
numbers of cups of coffee. In this case, there are several vation and experimentation to examine a horticultural
test groups, each of which receives the variable of caffeine. problem” shows how a young scientist used the process of
We could also include more than one control group. scientific inquiry to determine the cause of death in daf-
One could receive nothing to drink and the other a cup fodils, like the ones shown in FIGURE 0.7 , and to create

of water. In both cases, the control groups do not receive an effective treatment. This study also gives us a chance
caffeine. However, by providing a cup of water, we control to review how to use percentages when evaluating data,
for the potential variable of drinking. Both of these control described in “Analyzing Statistics and Data: Percent
groups are also called negative control groups because Change” on page 10.

the expectation is that we will see no effect. We could also

include a positive control group . This is a group that
receives a treatment or variable with a known result. In PREP FOR THE AP EXAM
®
®

our example, we could give a medicine that is known to AP EXAM TIP
increase heart rate to be sure that heart rate increases as you should know how to design a controlled experiment
expected. with a clear and precise hypothesis. the design should
If observations or experiments do not support a include experimental and control groups, and independent
hypothesis, the researcher modifies or rejects the hypoth- and dependent variables.
esis. If observations or experiments support a hypothesis,

8 Unit 1 chemIstry of LIfe

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