In the world of experimental study, a within subjects design, also known as repeated measures design, refers to experiments carried out with a particular group of participants, where the same participants are used for various experiments. This group is given various treatments (or is made to participate in various experiments), and these treatments are termed as independent variables. The repeated measures design is often used to carry out behavioral science experiments.
To explain what within subjects design is, let us consider the following example. An experiment is being carried out upon a group of participants, regarding the effects of food presentation over taste and vice versa. As such, they are first given a well presented meal followed by a delicious meal that is not so well presented. The participants are then asked to give their response, as to which meal they found better, the one that was well presented, or the one that was not so well presented. The experiment is carried out on the same group of people, so it is believed that the results will be more credible than those from an experiment carried out on varied groups of people.
Pros and Cons
As has been explained in the example above, within subjects design is a good choice because the scope for error and variance is very less. Since it is the same group of people that is being tested against several situations, it is likely that the results will be more conclusive. There will be a consistency in the responses elicited by the subjects of the group. The individual factors that affect the responses will be the same through all the experiments.
A special mention should be made about the difference between 'between subjects design' and 'within subjects design'. Between subjects design includes using various participants for different experiments, and not the same group. For instance, if we take the aforementioned example into consideration, let's assume that there were 14 participants in the experiment, all of whom were given the two meals. If it were a between subjects design, the participants would be divided into two groups of 7 each. This would affect the results elicited from the experiment, because both the groups were subjected to only one part of the experiment, and not both, so the opinions would vary. This is why the repeated measures design is considered to be more credible.
In terms of the cons, it is believed that the effect of one experiment on a group of participants is usually carried over to the next experiment, known as the carry over effect, thereby in some way tainting the results of the second experiment. Again, considering the example mentioned above, it is possible that after eating the first meal that was well presented, the participants may be full. This means, even if the second dish was delicious, the result after eating it may not be positive or pleasant, not only because they were full, but also because it was not presented well. This is known as the fatigue effect, when one experiment affects the other negatively. On the other hand, having the first meal may have warmed them up or built up their appetite to enjoy another meal. This is known as the practice effect, where the second experiment is positively affected by the previous experiment.
This is all that can be explained about the repeated measures design, in terms of its definition and its advantages / disadvantages.