What Is a Within-Subjects Design?

A within-subjects 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" describes the different levels of the independent variable, the variable that the experimenter controls. In other words, all of the subjects in the study are treated with the critical variable in question.

How to Use a Within-Subjects Design

When using a within-subjects design, it is important to make sure that all of the participants are exposed to the same treatment variables. By doing this, researchers can measure how each participant changes over time as a result of the treatment.

Example of a Within-Subjects Design

It can be helpful to look at some examples of how a within-subjects design might work. 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 most significant effect on performance on the memory tests.

What Is a 2x2 Within-Subjects Design?

A within-subjects design can also be a factorial design. A factorial design is a type of experimental design that can look at the effects of two or more independent variables.

In a 2x2 design, researchers examine how two independent variables with two different levels impact a single dependent variable. For example, imagine a study where researchers wanted to see how the type and duration of therapy influence treatment outcomes.

In a 2x2 design, they would examine two types of therapy (cognitive-behavioral and psychodynamic) and two levels of each treatment (short- and long-term).

How It Differs from a Between-Subjects Design

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 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.

When to Use a Within-Subjects Design

A within-subjects design can be a good option if participants or resources are limited. It can also be a good way to examine situations in real-world settings, such as to assess the effectiveness of educational programs.

When Not to Use a Within-Subjects Design

If researchers are concerned about the potential interferences of practice effects, they may want to use a between-subjects design instead. Within-subjects designs can also take more time to administer in some cases, so it may be helpful to use a between-sessions design if many participants are available to quickly conduct data collection sessions.

Advantages of Within-Subjects Design

There are a few different advantages to using a within-subject design when conducting a psychology experiment.

Uses a Smaller Sample Size

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 requires twice as many participants as a within-subject design when two or more groups of participants are tested with different factors.

Reduces Errors Caused by Individual Differences

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.

Carryover Effects

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 impact their later performance in jogging and may even affect their performance on later memory tests.

Participant Fatigue

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

Practice Effects

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 to do better in 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.