7.6: Within Subjects Design

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Page ID: 124578

Rajiv S. Jhangiani, I-Chant A. Chiang, Carrie Cuttler, & Dana C. Leighton
Kwantlen Polytechnic U., Washington State U., & Texas A&M U.—Texarkana

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Learning Objective

Describe and give an example of a within-subjects design.

Within-Subjects Design

In a within-subjects, each participant is tested under all conditions. To gather subjects for this type of experiment you would use a non-probability sample. Consider an experiment on the effect of a defendant’s physical attractiveness on judgments of his guilt. Again, in a between-subjects experiment, one group of participants would be shown an attractive defendant and asked to judge his guilt, and another group of participants would be shown an unattractive defendant and asked to judge his guilt. In a within-subjects experiment, however, the same group of participants would judge the guilt of both an attractive and an unattractive defendant.

Here is what this experiment type would look like.

X1 01 X2 01

O¹: First post-test

O²: Second post-test

X¹: Experimental Stimulus

X²: Control Stimulus or

Comparison Stimulus

The primary advantage of this approach is that it provides maximum control of extraneous participant variables. Participants in all conditions have the same mean IQ, same socioeconomic status, same number of siblings, and so on—because they are the very same people. Within-subjects experiments also make it possible to use statistical procedures that remove the effect of these extraneous participant variables on the dependent variable and therefore make the data less “noisy” and the effect of the independent variable easier to detect. We will look more closely at this idea later in the book . However, not all experiments can use a within-subjects design nor would it be desirable to do so.

KEY TAKEAWAY

This type of research is useful for comparisons.