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Formulating Hypotheses : “If…then…because…”
Science advances by formulating and testing hypotheses related to a specific problem or
testable question. A hypothesis is really a temporary explanation about what will happen
under certain conditions. For example, you might say, "If green plants need light to grow,
then only plants kept in light will display growth, because..." This statement is a
hypothesis. When a scientist designs an experiment, he or she makes an "if.....,"
statement. By thinking about what kinds of observations could be made to support or
disprove it, the scientist can make a "then" statement. The “then” statement predicts what
will happen if the "if" statement is true. Typically the “if” part of the statement involves
the assumption that one variable affects another. The “then” part of the statement
involves the actual prediction. Finally, the “because” portion involves the scientist’s
explanation. In the example above, the “because” might be something like: “because
plants use light to build sugars.”
First identify the two variables involved, the manipulated (underline once) and
responding (underline twice). Second, begin writing the hypothesis by substituting the
two variables in the blanks:
a. If the __(m.v.)__affects__(r.v.)__,
b. then ____. Following the “then” is your prediction concerning what happens to the
responding variable when specific changes are made to the manipulated variable. If
possible, include at least two specific control variables (c.v.) within your hypothesis.
c. ‘because’. Following the “because” is your predictive reason. It is your explanation
why the responding variable changes as a result of the changes made to the manipulated
variable.
To help you learn how to form hypotheses, arrange the following examples in "if...,
then...because…" format.
1. A scientist thinks that living things give off carbon dioxide gas as they break down
food. He or she predicts that carbon dioxide can be detected as an organism digests its
food.
2. Some students were investigating what made their classmates get colds. One student
believed that those who took Vitamin C everyday got fewer colds than the rest of the
students.
3. Another student was interested in raising tropical fish. She believed that fish that were
given brand X food would produce more offspring than those who were fed brand Y.
There is more that you should know about formulating hypotheses and making
predictions. You may find that your prediction was supported by the data. But this result
does not necessarily prove that your hypotheses will be correct for all similar situations.
For example, a scientist might observe hundreds of different kinds of living things. Each
time, he or she may find that carbon dioxide as is given off as food is broken down.
Eventually, though, one experiment may show that some kind of organism does not give
off carbon dioxide. Then the hypothesis that all organisms give off carbon dioxide is not
true; it is not supported by the data.
Thus, scientists can disprove hypotheses but cannot prove them to be true. A
hypothesis may not be proven to be true in all cases. There is always the possibility that
an exception to the hypothesis will be found. Even so, the more experiments there are that
support a hypothesis, the greater its power to explain a phenomenon. Eventually,
scientists may generate a theory regarding the matter at hand. A theory is an explanation
or model for a natural phenomenon that is supported by a large amount of evidence.
For each of the following problems/questions involving bacteria, formulate a hypothesis.
First, identify the manipulated (underline once) and responding variables (underline
twice). Second, write and “If…then…because…” statement as you did in questions 1-3.
4. Does the temperature at which bacteria are grown affect their rate of reproduction?
5. Does using up the nutrients they are feeding on affect the rate of reproduction of
bacteria?
6. Some bacteria require certain vitamins in the material in which they are grown. Does
bacterium X require vitamin Y for reproduction?
7. Does aspirin inhibit (stop) the reproduction of bacteria?
8. You have had some practice in forming hypotheses and making predictions. Now ask a
question of your own about some living thing. Form a hypothesis and make a prediction
about your question.
9. Is it possible for a biologist to prove your hypothesis? Why or why not?
10. Is it possible for a student’s prediction to be supported by the data and yet be wrong
in the explanation (the ‘because’ portion of the hypothesis)? Explain.