Within Subjects Design and Between Subjects Design

A within-subject design is a type of experimental design in which all participants are exposed to every treatment or condition. It is also known as a repeated measures design.

The term "treatment" is used to describe the different levels of the independent variable, the variable that's controlled by the experimenter. In other words, all of the subjects in the study are treated with the critical variable in question.

This article discusses what a within-subjects design is, how this type of experimental design works, and how it compares to a between-subjects design.

Within-Subjects Design vs. Between Subjects

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 the participants try yoga before taking a memory test. Then, you have all the 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.

This within-subjects design can be compared to what is known as a between-subjects design. In a between-subjects design, people are only assigned to a single treatment. So one group of participants would receive one treatment, while another group would receive a different treatment. The differences between the two groups would then be compared.

Consider the earlier example of the experiment looking at exercise and memory. In a between-subjects design, one group of participants would do yoga and then take a memory test. A different group of participants would jog and then take the memory test. Afterward, the results of the memory tests would be compared to see how the type of exercise influenced memory.

Advantages of Within-Subjects Design

Why exactly would researchers want to use a within-subject 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-subject design, which is when two or more groups of participants are tested with different factors, would require twice as many participants as a within-subject design.

A within-subject design can also help reduce errors associated with individual differences. In a between-subject design where individuals are randomly assigned to the independent variable or treatment, there is still a possibility that there may be fundamental differences between the groups that could impact the experiment's results.

Disadvantages of Within-Subjects Design 

This type of experimental design can be advantageous in some cases, but there are some potential drawbacks to consider. A major drawback of using a within-subject design is that the sheer act of having participants take part in one condition can impact the performance or behavior on all other conditions, a problem known as a carryover effect.

So for instance 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.

Finally, performance on subsequent tests can also be affected by practice effects. Taking part in different levels of the treatment or taking the measurement tests several times might help the participants become more skilled.

This means they may be able to figure out how to game the results in order to do better on the experiment. 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.