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.
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
One of the greatest advantages of a within-subjects design is that it does not require a large pool of participants. Generally, 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 the results will not be distorted by individual differences. Each participants serves as his or her own baseline.
Drawbacks of a Within-Subjects Design
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 carryover effects. In our earlier example, having participants take part in yoga might have an effect on their later performance in jogging and may even impact 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.
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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