Extraneous & Confounding Variables: Differences & Examples

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  • 0:06 Internal Validity
  • 1:45 Extraneous Variables
  • 4:21 Confounding Variables
  • 6:00 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

What happens when something other than your independent variable is influencing the outcome of your study? In this lesson, we'll look at two types of variables that can affect an experiment: extraneous and confounding variables.

Internal Validity

Josh is in love. He's been with his girlfriend a while now and wants to propose. But he doesn't know how he should do it. Should he propose in a crowd? When they're alone? At the place where they went for their first date? After he whisks her off to Paris or the Bahamas?

Josh is a psychologist and does research for a living, so he decides to do a study on marriage proposals and figure out which one women like best. That's how he'll decide how to propose. He gathers a bunch of women, shows them videos of marriage proposals, and then measures their reactions: whether they cry or if their heart races or if they just watch it and go, 'Eh.'

In research, internal validity is when a researcher can say that only the independent variable caused changes in the dependent variable. For example, in Josh's study, the videos are the independent variable and the women's reactions are the dependent variable. If Josh changes which videos he shows the women, he sees different reactions. If his internal validity is high, he can say that the difference in videos caused the changes in the reactions.

If most women who watch video A say, 'Aw, how sweet!' and most women watching video B say, 'Well, that's an epic fail,' then Josh wants to know for sure that it's actually the video that's causing the reactions, not something else. Let's look closer at variables that might affect the dependent variable besides the independent variable: extraneous and confounding.

Extraneous Variables

Okay. So, let's imagine that Josh has set up his experiment. Each subject is brought into a little room and is shown two of six different videos. Josh measures their reaction to each video and then their reaction overall.

Josh expects that he will see the women react more positively to the videos they believe are most romantic. Not only that, but he believes that if he shows a woman two proposals that most women believe are really romantic, then she'll have a higher reaction level overall than someone who is shown only one really romantic video and one that's, well, sort of romantic. But what happens if the women who are shown two really romantic proposal videos are put in a room that's much warmer than the other women? Or what if they are given a red rose before going into the room but the other women aren't?

Both of these are examples of extraneous variables, or variables present in the experiment that aren't being studied. If all of the women who are shown the two most romantic proposals are tall and all the other women are short, will that make a difference? What about the examples we gave above of room temperature or the rose? How will those affect the outcomes of the study?

The problem with extraneous variables is that they might affect the dependent variable but they might not. There's no way to tell until after the experiment is done. Extraneous variables are usually grouped into three categories:

  1. Physical or Situational Variables: These occur when the physical situation of subjects changes for certain groups, like the fact that women shown the most romantic proposals are in a warmer room.
  2. Personal Variables: These are when one group has personality or other traits that members of the other group don't. For example, what if the women shown the most romantic video clips are also more romantic in nature than the other women?
  3. Researcher Variables: These are when the researcher, himself, does something different for the various groups of the experiment. For example, what if Josh was really nice to the women who saw the two romantic videos, and he was very gruff with the other groups?

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