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Drawing Conclusions Based on Internal Validity

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  • 0:03 Internal Validity
  • 1:38 Extraneous Variables
  • 4:23 External Validity
  • 7:10 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.

When a researcher gets the results of their study back, how do they know that the independent variable caused the results? In this lesson, we'll look at how internal validity shapes the way researchers draw conclusions about their research.

Internal Validity

Brett is a psychologist who has done a study to look at how images of extremely thin models affect the self-esteem of female college students. He divided his subjects into two groups. He gave the first group a magazine with a bunch of very thin models in it. He gave the other group a magazine with normal and plus-sized models in it.

After the subjects had looked at the magazine, he gave them a questionnaire to see how much self-esteem they had and how confident they were about their own bodies. When he looked at the results, he noticed that the subjects who looked at the magazines with skinny models had much lower self-esteem and were less confident about their bodies than the other subjects.

That's it then, right? Brett's done his study, and his results show that his hypothesis is correct. Time to pat himself on the back and move on, right? Well, not exactly. You see, Brett still has to figure out whether his conclusions are valid or not. One element that he needs to look at in his study is internal validity, or the ability of the researcher to say that only the independent variable caused the changes in the dependent variable.

In Brett's case, his independent variable was which magazine photos the subjects looked at. His dependent variable was their self-esteem and body confidence. He wants to be able to say that it was the photos of the models in the magazine and nothing else that caused one group to have lower self-esteem and body confidence than the other group. Let's look at how Brett can analyze his internal validity to see if his results are valid.

Extraneous Variables

Okay, so Brett needs to figure out how strong the internal validity of his study is. But what exactly does that mean? Remember that internal validity is the extent to which Brett can say that his independent variable (types of models in the photos) caused the changes in the dependent variables (self-esteem and body confidence) and nothing else. For example, what if the subjects who were in the skinny model condition already had lower self-esteem and body confidence than those who were in the other condition? In other words, maybe they came into the study with lower self-esteem, and the results reflect that.

Or perhaps everyone started out with the same self-esteem levels, but something else changed the self-esteem of one group or another. For example, maybe there was an article in the plus- and normal-sized magazine that was all about feeling confident. After reading that article, the subjects in that condition had higher self-esteem and body confidence not because of the photos, but because they were exposed to that article.

In both those cases, Brett's results were the result of an extraneous variable, or a variable other than the independent variable that might affect the dependent variable. In Brett's case, the subjects' baseline for self-esteem and the article about confidence are both extraneous variables that could be causing his results.

As you can probably guess from their definitions, internal validity and extraneous variables are linked to one another. If there are a lot of extraneous variables present, the researcher is not able to say for sure that the independent variable is the only variable affecting the dependent variable. As a result, the presence of extraneous variables lowers internal validity.

What can Brett do? Well, after the study is done, there's not a lot that he can do about extraneous variables. That's why most researchers spend time near the beginning of the study thinking about the extraneous variables that they might run into. Once they can identify the extraneous variables, they try to eliminate their effects as much as possible.

For example, Brett could take a measurement of all of the subjects' self-esteem and body confidence at the beginning of the study. That way, he could make sure that both groups are approximately equivalent on those measures and one group doesn't start the study with higher or lower self-esteem. He could also make sure that the two magazines have the exact same text in them. If only the photos are different, he could draw the conclusion that his internal validity is high, and, therefore, that results are the result of the photos and nothing else.

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