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Pretest-Posttest Design: Definition & Example

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  • 0:07 Definition
  • 1:38 Example 1
  • 4:00 Example 2
  • 5:35 Lesson Summary
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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught Psychology and has a master's degree in Clinical Forensic Psychology, and will earn a PhD in 2015.

This lesson explores the process and technique of using a pretest-posttest design in psychology. Two simple examples will allow you to understand how to apply this type of design in a future experiment.

Definition

You've probably got some ideas of how experiments should be run. Why don't researchers just look at something, poke it with a stick, and then study the changes? Researchers are always making things super complicated.

I am glad to inform you that there is a methodology very similar to this, most of the time occurring without the stick. A pretest-posttest design is usually a quasi-experiment where participants are studied before and after the experimental manipulation. Remember, quasi-experimental simply means participants are not randomly assigned. It is possible to have a control group, or a group who doesn't receive the manipulation, but we will not be looking at that in this lesson. In a pretest-posttest design, there is only one group and all of them are in the experimental condition.

The reason you run a pretest-posttest experiment is to see if your manipulation, the thing you're looking at, has caused a change in the participants. Since everyone is being manipulated in the same way, any changes you see across the group of participants is likely from the manipulation. This means you test them before doing the experiment, then you run your experimental manipulation, and then you test them again to see if there are any changes. So how does this really work?

Example 1

Have you ever tried to go about your day when you haven't showered, brushed your teeth, or really cleaned yourself? Let's say you're a researcher who is interested in how much the feeling of being unclean affects judgment and general knowledge. You settle on a pretest-posttest design. You will administer a pretest on general knowledge and judgment, then have your experimental manipulation of the participants not cleaning themselves, then perform a posttest using the same or similar tests.

You collect your participants and give them a series of tests that will measure their judgment and general knowledge. For example, you may look at their ability to discern when it would be a good time to pull out into traffic as a measure of judgment. For general knowledge, you write up a short test about historical, scientific, and literary ideas.

Next, you instruct all of your participants to not shower, brush their teeth, or clean themselves for 36 hours. This is to ensure maximum nasty grossness in your study. After 36 hours, your participants return.

You likely would not give them the exact same tests because there's a practice effect, defined as an influence on performance from previous experience. So if you have them do the exact same car driving sequence, they may have learned a pattern or become familiar with the machine. If you give them the exact same general knowledge test, then they may have looked up some of the answers or had a chance to remember more.

Lastly, you will compare the participants' pre- and posttests to see if being dirty affected their judgment or general knowledge. The purpose of this experiment was to demonstrate if there was a change in the participants brought on by your experimental manipulation. If the posttest is significantly different from your pretest, then your manipulation caused some kind of change. Remember, this is a quasi-experimental study since all of the participants went 36 hours without cleaning. This means you don't have randomly assigned experimental and control groups. If you wanted a true experiment, then you would need additional steps to randomly sort your participants.

Example 2

Therapy is often something that is tested using pretest and posttest design. Let's say you have designed a new type of therapy for treating anxiety. This type of study would involve testing your participants' anxiety levels, providing them with therapy as the experimental manipulation, and then testing their anxiety levels again.

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