What Is a Within-Subjects Design?

Overview, Advanges and Disadvantages of Within-Subjects Design

A within-subjects experiment
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A within-subjects design is a type of experimental design in which all participants are exposed to every treatment or condition. The term treatment is used to describe the different levels of the independent variable. In other words, all of the subjects in the study are treated with the critical variable in question.

So, for example, let's imagine that you are doing an experiment on exercise and memory.

For your independent variable, you decide to try two different types of exercise: yoga and jogging. Instead of breaking participants up into two groups, you have all participants try yoga before taking a memory test. Then, you have all participants try jogging before taking a memory test. Next, you compare the test scores to determine which type of exercise had the greatest effect on performance on the memory tests.

Advantages of a Within-Subjects Design

So why exactly would researchers want to use a within-subjects design? One of the most significant benefits of this type of experimental design is that it does not require a large pool of participants. A similar experiment in a between-subjects design would require twice as many participants as a within-subjects design.

A within-subjects design can also help reduce errors associated with individual differences. In a between-subjects design where individuals are randomly assigned to a treatment condition, there is still a possibility that there may be fundamental differences between the groups that might impact the results.

In a within-subjects design, individuals are exposed to all levels of a condition, so individual differences will not distort the results. Each participant serves as his or her own baseline.

Drawbacks of a Within-Subjects Design

This type of experimental design can be advantageous in some cases, but there are some drawbacks to consider.

A major drawback of using a within-subjects design is that the sheer act of having participants take part in one condition can impact performance or behavior on all other conditions, a problem known as a carryover effect. In our earlier example, having participants take part in yoga might have an impact on their later performance in jogging and may even affect their performance on later memory tests.

Fatigue is another potential drawback of using a within-subjects design. Participants may become exhausted, bored or simply disinterested after taking part in multiple treatments or tests.

Finally, performance on subsequent tests can also be impacted by practice effects. Taking part in different levels of the treatment condition or taking the measurement tests several times might help the participants become more skilled. This can skew the results and make it difficult to determine if any effect is due to the different levels of the treatment or simply a result of practice.

Also Known As: Repeated-measures design

References:

Hall, R. (1998). Within-subjects design. Psychworld. Found online at http://web.mst.edu/~psyworld/within_subjects.htm

Shuttleworth, M. (2009). Within subject design. Found online at http://www.experiment-resources.com/within-subject-design.html

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