Two-Group Experimental Designs: Definition & Examples

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  • 0:06 Two-Group Design
  • 1:10 Groups
  • 1:59 Assigning Groups
  • 3:27 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.

How does a researcher know if their treatment has an effect or not? In this lesson, we'll look at two-group experimental designs, contrast control and treatment groups, and examine random assignment and matched groups.

Two-Group Design

Rory is a psychologist, and he is interested in the effect of watching a popular science fiction show. He wants to know if watching the show will cause people to believe more in aliens than if they don't watch the show.

Experimental design is the process by which a researcher decides how to run a study. For example, Rory might decide to get a bunch of subjects and divide them into two groups. He presents the show to one group and doesn't present it to the other group. Afterward, he asks whether or not they believe in aliens. If the group who watched the show answers 'yes' more often than the group that didn't, he knows that watching the show will increase belief in aliens.

Rory has chosen a two-group design, which is when an experiment is done on two groups of subjects and the results are then compared. For example, Rory is going to compare the belief in aliens of two groups: those who watched the show and those who didn't. Let's look closer at the decisions Rory has to make in order to make his two-group design work.


A two-group design almost always involves a control group and an experimental, or treatment, group. The control group does not get the treatment, while the treatment group does get the treatment (hence its name). In Rory's case, the treatment is the television show, so the treatment group is the group that watches the show. The control group is the group that does not watch the show.

In a simple, two-group design with a control group, the researcher wants to know whether the treatment has an effect or not. Rory, for example, wants to know if watching the television show will have an effect on belief in aliens. His alien-belief survey will give him an idea of how much each group believes in aliens; and if the group who watched the show believe more, he can draw conclusions about the effect of the show.

Assigning Groups

But Rory still has to decide how to assign his subjects to each group. Most researchers use random assignment, which means that they put participants in groups using a random method. Maybe Rory flips a coin to see if someone will be in the control or treatment group. Maybe he draws names out of a hat. There are also online randomizers that can help Rory assign people to groups.

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