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Lesson 2: What Is an Experiment?

Lesson 2: What is an Experiment?


Students will learn about the elements of an experiment and the meaning of "causation". Students will learn to distinguish claims of causation from claims of association.


  1. Video: MythBusters’ Is Yawning Contagious?

    Note: Please use a search engine (e.g., Google Video) and type “MythBusters Is Yawning Contagious” to find it. The clip is a little over 5 minutes in length. If you cannot access it, an alternate experiment (MythBusters' How Does Music Affect Plants) can be found at


experiment subjects treatment treatment group control group random assignment outcome research question confounding factors

Essential Concepts:

Essential Concepts:

Science is often concerned with the question "What causes things to happen?". To answer this, controlled experiments are required. Controlled experiments have several key features: (1) there is a treatment variable and a response variable, and we wish to see if the treatment causes a change that we can measure with the response variable; (2) There is a comparison/control group; (3) Subjects are assigned randomly to treatment or control (randomized assignment); (4) Subjects are not aware of which group they are in (a 'blind'). This may require the use of a placebo for those in the control group; and (5) those who measure the response variable do not know which group the subjects were in (if both 4 and 5 are satisfied, this is a 'double blind' experiment).


  1. Display the following headlines to students:

    1. Stop Global Warming: Become a Pirate

    2. Lack of sleep may shrink your brain

    3. Early language skills reduce preschool tantrums

    4. Dogs walked by men are more aggressive

  2. Discuss each headline by asking the following questions:

    1. What is the headline implying with its wording? 1a is implying that you can stop global warming by becoming a pirate, 1b is implying that it’s possible to shrink your brain if you aren’t getting enough sleep, 1c is implying that having early language skills will decrease preschool tantrums, 1d is implying that dogs are more aggressive when they’ve been walked by men.

    2. Is it implying causation or association? Discuss definitions of causation and association. Causation means there is a cause and effect relationship between variables. For example, heat causes water to boil; whereas association or correlation means that high values of one variable tend to be associated with high values of the other (or high values tend to be with low values). However, this is not necessarily cause-and-effect at play. For example, blanket sales in Canada are associated with brush fires in Australia - not because Canadian blankets cause the fires, but because Canadian winters cause blanket sales, and Canadian winters are Australian summers, which cause fires. 1a, 1c and 1d are implying causation and 1b is implying association.

    3. How can you tell the difference between causation and correlation? What words stand out in these headlines? Answers will vary but some terms for causation include: cause, increase/ decrease, benefits, impacts, effect/ affect, etc.; and for correlation include: get, have, linked, more/ less, tied, connected, etc. In 1a, “become” stands out; in 1b, “may” stands out; in 1c, “reduce” stands out; in 1d, “are” stands out.

    4. Change each causal version of a headline into a non-causal version and vice versa. Answers will vary but an example for 1a is to instead say Global Warming linked to increase of pirates.

  3. Introduce the MythBusters video clip by answering the following questions, in teams, for their headline “Is Yawning Contagious?”

    1. What is the headline implying with its wording? That yawning may cause other people to yawn.

    2. Is it implying causation or correlation? How do you know? Causation because “contagious” yawns means that you are yawning because someone else has yawned.

    3. How can we determine if this is true? Split the class into groups and have each team come up with a way to determine if this is true. Each group should assume that they get to examine 50 people.

  4. Show the MythBusters video clip called Is Yawning Contagious? The links to the clips can be found in the Materials above.

  5. Focus students on the following guiding questions and ask them to take notes as they watch the video clip:

    1. How did the MythBusters design the investigation?

    2. What steps did they take?

    3. How is this different than your team’s headline responses?

  6. After viewing the clip, inform students that the MythBusters have just conducted an experiment, which is one method of data collection.

  7. We begin with a brief introduction into “what is an experiment” but the definition will be developed over the next several lessons.

  8. Guide students to identify the elements of an experiment by referring back to the video clip:

    1. Research Question — the question to be answered by the experiment (Is Yawning Contagious?)

    2. Subjects – people or objects that are participating in the experiment (the 50 adults)

    3. Treatment – the variable that is deliberately manipulated to investigate its influence on the outcome; this is sometimes known as the explanatory, or independent, variable (Kari yawned before subject entered the room)

    4. Treatment group – the group of subjects that receive the treatment (two out of every three subjects who were placed into rooms – yawn from Kari)

    5. Control group – the group that does not receive a treatment (one out of every three subjects who were placed into rooms – no yawn from Kari)

    6. Random assignment – subjects are randomly assigned to either the treatment or control group (two out of every three subjects received the treatment - in this experiment, random assignment was not used (or if it was, we were not told so))

    7. Outcome – the variable that the treatment is meant to influence; this is sometimes known as the response, or dependent, variable (whether or not a person yawned)

    8. Statistic — a method for comparing the outcomes of the control and treatment groups (this is needed) In this case, the MythBusters used the difference between the percent of subjects that yawned in the treatment group, which was 4% higher than the control group.

    Note: In this experiment, and in those found in the IDS curriculum, we use a treatment and a control group. However, a control group is not a necessary element of an experiment. Sometimes it is more appropriate to have two treatment groups with no control group (e.g., medical professionals testing different doses of drugs). The effect that is being studied will dictate whether to feature a control group or not.

  9. Display the following questions on the board or projector. Using T-I-P-S, ask students to discuss them.

    1. Why did the MythBusters follow all of these steps to design their experiment? In order to determine if watching someone yawn can cause you to yawn.

    2. We don't know how MythBusters chose who would be in the treatment group and who would be in the control group. Suppose that the people who showed up first, early in the morning, were assigned to the treatment group, and the last few people, later in the day, ended up in the control group. Would you believe in the conclusions? No, because the two groups were different. The first group might have been sleepier, and so more likely to yawn anyways. Explain that this --another explanation for the cause-and-effect-- is called a confounding variable.

    3. Explain that in order to make the two groups as similar as possible, experimenters usually assign subjects randomly. How might we randomly assign about half of the subjects to the treatment and half to the control? We might flip a coin, and those who get Heads go to Treatment.

    4. Why would random assignment improve the MythBusters experiment? Because then the two groups would be more similar, so we wouldn't have a confounding variable to worry about.

  10. Emphasize that without random assignment, we cannot determine causation because we are not comparing two similar groups.

Class Scribes:

One team of students will give a brief talk to discuss what they think the 3 most important topics of the day were.