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 Steps
Introductory Activity:
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Share the following scenario with your students: A
runner has two pairs of running shoes. To find out which pair will
allow her to run faster, she puts on the first pair and runs around
the track once in exactly 2 minutes. She puts on the second pair and
runs around the track again, this time, making it in 2 minutes and
10 seconds. She decides the first pair is better, and heads to the
store to return the second pair. Ask students:
- What is the runner trying to test? That is, what question is
she trying to answer?
- Do you think her experiment can answer the question she is trying
to answer? Why or why not?
- Do you think both pairs of sneakers are treated fairly in the
experiment or does one have better chances than the other? Why
or why not?
- Are there any factors that might have interfered with her experiment?
If so, what are they? (e.g., what if she is winded from running
around the track the second time?)
- What did the runner assume when she designed this experiment?
(Answer: that the type of shoes she was wearing would create a
difference one way or the other.) Note: If students can’t
come up with an answer for this question, have them think about
it for homework. They will revisit it in the next activity.
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Record students’ answers on a piece of chart paper.
Tell them that in following activities, they are going to use what
they learn to design a better experiment for the runner.
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Learning Activities:
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Activity 1
Part one-independent, controlled, and dependent variables
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Ask students to brainstorm what the following terms
mean: independent variable, dependent variable, extraneous variables,
controlled variables, and control group.
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Then, write these simple definitions on the board:
- Independent variable: the variable that is purposefully changed
in an experiment
- Controlled variables: the variables that are kept the same during
an experiment
- Dependent variable: the variable that is changed depending on
the value of the independent variable
- Extraneous variables: variables that are not or cannot be controlled
that could interfere with an experiment
- Control group: a group in which everything is the same as the
test group except that the independent variable stays the same
You may want to print out a copy of the following Web page to further
explain these concepts:
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Tell students they can begin to understand these terms
by thinking back to the runner scenario. Break students into pairs
and have them discuss and answer the following questions. (Go to STUDENT
ORGANIZER: ACTIVITY ONE-PART ONE for a printer-friendly version
of the following questions that doesn’t contain answers.)
- Which variable changed (independent variable) in this experiment?
(the running shoes the runner was wearing)
- What were some of the controlled variables that should have
stayed the same? (her energy level, the distance she ran)
- Did all the controlled variables actually stay the same? (e.g.,
energy level) If not, how could they affect the experiment? (she
may have been more tired the second time around, the wind may
have picked up, she may not have tied the second pair as tightly
as the first, etc.)
- Was there a control group in the experiment? If so, what was
it? If not, should there have been one? Why or why not?
- What was the dependent variable in this experiment supposed
to be? (the length of time it took the woman to run around the
track)
- In a successful experiment, the dependent variable must change
based on changes in the independent variable. Is this the case
with the runner’s experiment? Did the dependent variable
(the time it took the woman to run around the track) change based
solely on the independent variable (the shoes the woman was wearing),
or were other variables possibly involved?
- Based on what you know, was the runner’s experiment successful?
Do you think she made a good decision about which shoes to keep?
Why or why not?
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Bring students back together to discuss the answers
to the questions above. Record a list of what the dependent, independent,
extraneous, and controlled variables were in the experiment.
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HOMEWORK: Read through the STUDENT
ORGANIZER-ACTIVITY ONE HOMEWORK assignment with students to make
sure they understand it.
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Part two-control groups
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Remind students about the runner scenario from part
one of this activity. Ask about the existence of a control group.
There wasn’t one in the runner’s experiment, but there probably should
have been. If students have trouble understanding the concept of control
groups, use the following anecdote to help them understand:
Imagine that the runner’s experiment wasn’t flawed. Instead, the runner
had controlled all of the variables in her running experiment. Every
time she wore the first pair of sneakers, she ran around the track
in 2 minutes. Each time she ran around the track in the second pair
of sneakers, she ran around the track in 2 minutes and 10 seconds.
Happy with what she thought were such decisive findings, she bought
the first pair of shoes. What assumption did she make?
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If students have difficulty coming up with the answer,
tell them: The woman is assuming that whichever running shoes she
is wearing will actually make a difference in her running speed. The
woman’s running speed may vary + or -10 seconds no matter what. Or,
her speed could vary independently of the shoes she wears, so the
type of shoes she was wearing may not have made any difference in
her running speed.
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Ask students: Did the runner’s assumption affect the
way she designed her experiment? How? (Because she is assuming that
the type of shoes she is wearing will make a difference, she is attributing
any difference in her running speed to the shoes.) Tell students that
this type of assumption, based on personal opinion, is called a bias,
and that one of the greatest challenges scientists face is trying
to identify these biases in their experiment designs.
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Break students into pairs. Ask them to discuss the
following question: How could the runner design an experiment to avoid
this design bias? In other words, how can she be sure that running
shoes actually affect running speed, and that wearing one pair or
another will impact your pace? (One way is by using a control group.)
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Reconvene as a group and record students’ ideas on
the board.
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Part three—designing a better experiment
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Break students into small groups and ask them to design
an experiment for the runner that would help her make a sounder decision
regarding the sneakers. Remind them that to do this, they’ll need
to eliminate all the variables except the running shoe variable. Tell
them to use STUDENT ORGANIZER:
ACTIVITY ONE-PART THREE to help design their experiment.
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Group by group, have students share their revised experiments
with the class. For each experiment, analyze whether all but the running
shoe variable were controlled.
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If the students missed the idea of repetition as a
way to gather reliable data in their responses, you may want to share
the following example:
Your parent or guardian asks you to time your walk to
school—he or she wants it to be as accurate as possible. If you
time it only once, your estimate may not be representative. Maybe
you were tired that day, and walked slower than usual. Maybe it
was hot out, and you crossed the street a few times to stay in the
shade, adding to the length of your walk. Maybe you were late so
you ended up running. But, if you time yourself several days in
a row, the differences caused by some of these circumstances start
to even out. On the first day you walk slower than usual, and it
takes you 15 minutes. The second day you’re really late, so you
run and it takes you 10 minutes. The third day, you walk at a regular
pace, and arrive after 13 minutes. If you average your results,
they will be more accurate than if you just take one measurement.
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If possible, choose the experiment design that seemed
the soundest, or combine elements from the different experiments that
seemed effective to create an experiment design that all of the students
are happy with.
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Then, have the students use two pairs of running shoes
to try the experiment. They don’t both have to be new pairs—one could
be new, one could be old. Have students compare the actual results
with the results they had expected.
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Activity 2
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Now, tell the students that they’re going to conduct
an experiment to learn more about the role of independent, dependent,
extraneous, and controlled variables in scientific experiments.
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Pose the following question to the students: If you
roll a pencil from an incline, will it roll twice as far as a pencil
rolled from half as high up the incline? For example, if you roll
a pencil from 2 feet up an incline, will it roll twice as far as a
pencil rolled from one foot up the incline?
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Break students into groups of three to four. Give each
group STUDENT ORGANIZER-ACTIVITY
TWO. Ask them to complete all sections of this sheet.
Note to teachers: You may want to have students assume specific roles
within their groups: data recorders (to take measurements and record
the data), variable controllers (to make sure that each repetition
of the experiment is consistent and avoids the pitfalls discussed
earlier), and engineers (to set up the plank, roll the pencil).
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After conducting the experiment, ask students to discuss
the data and analysis they recorded on their STUDENT
ORGANIZER-ACTIVITY TWO handout.
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As a class, ask students to generate a list of designing
and experimenting Do’s and Don’ts. Record their list on a piece of
chart paper and display in the classroom.
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Culminating Activity/Assessment:
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For the final activity of this lesson, student
will share what they’ve learned with another classroom. They can do
this by creating a one-hour, student-led workshop for another class
in their school, or create a Web page that shares what they’ve learned.
Their presentation or Web page should include the following:
- Definitions of the terms “independent”, “dependent”, “controlled”,
and “extraneous variables”, and “control group”.
- An explanation of why these terms are important in a scientific
experiment, along with examples that illustrate their points.
- A practical list of Do’s and Don’ts of experiment design.
- An activity that will help another class understand the new
concepts.
Note to teachers: Use the following Web sites for information on how
to create a PowerPoint presentation or Web page:
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Extensions
Cross-Curricular Extension: History
Have students go to the interactive Web site listed below to learn
how yellow fever was cured. After reading through the site, have students
analyze the events in terms of the scientific method. Have them determine
what the question the scientists were trying to answer, the method
they used to uncover the truth (including the dependent, independent,
controlled, and extraneous variables—they will have to infer
what these were) and what the results of the experiment were.
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