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Controlling for Extraneous Variables: Single Blind, Double Blind & Placebo Methods

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Instructor: Natalie Boyd

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

In research experiments, extraneous variables can interfere with the research process, making it unclear whether a dependent variable is affected by the independent variable(s), extraneous variable(s), or both. Review examples of extraneous variables, and learn ways to control for their impact in experiments, including single blind, double blind, and placebo research methods. Updated: 09/27/2021

Extraneous Variables

Hannah is a psychologist, and she has found a new drug that she thinks will help treat depression. To help prove that the drug works, she decides to do a research study on it. In Hannah's study, her independent variable is the drug, and her dependent variable is the level of depression experienced by the subjects. She wants to show that her independent variable (the drug) causes a decrease in her dependent variable (depression).

That sounds pretty straightforward at first. All Hannah has to do is to give her drug to a bunch of depressed people, and if they feel better, she's proved her point, right? Well, it might not be so simple. There are a lot of things that could cause people to be less depressed. What if the people Hannah gives the drugs to start to feel better for some reason other than the fact that they got the drug?

Extraneous variables are factors other than the independent variable that might affect the dependent variable. For example, maybe the people to whom Hannah gives the drugs think, 'Since she's giving me the drugs, I'm going to start feeling better.' Maybe, in anticipation of feeling better, they start exercising and eating better. These are things that could affect depression levels. So, maybe they're less depressed at the end of the study because of their lifestyle changes, not Hannah's drug. Let's look at some ways to control for extraneous variables.

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  • 0:06 Extraneous Variables
  • 1:36 Single-Blind Study
  • 3:24 Double-Blind Study
  • 4:41 Lesson Summary
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Single-Blind Study

Okay, so Hannah wants to design a study to prove that her new drug can treat depression. She decides that she needs a treatment group made up of subjects who get the treatment (or drug) and a control group made up of subjects who do not get the treatment (or drug). Then she'll compare how much their depression gets better. If she's right that her drug works, the people in the treatment group will show more improvement than those in the control group.

But, there's a problem with that: if she gives the treatment group a pill and doesn't give the control group anything at all, the results might not be reliable. This could be for a couple of different reasons. First, the people in the treatment group might tell Hannah what they think she wants to hear; they'll say, 'Yeah, your drug works. I feel much better.'

Another reason that the results might not be reliable is called the placebo effect. This is when people feel better because they are given a drug, not because the drug works, but because they believe the drug will work. It's a case of mind over matter: their brain tricks them into feeling better. This is called the placebo effect because it occurs when subjects are given a placebo, or a pill that doesn't do anything. When subjects are given one of the placebo pills, they begin to feel better.

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