Advantages of Within-Subject Designs (1 of 2)

Subjects inevitably differ from one another. In an experiment on children's memory, some children will remember more than others; in an experiment on depression, some subjects will be more depressed than others; in an experiment on weight control, some subjects will be heavier than others. It is simply a fact of life that subjects differ greatly. In between-subject designs, these differences among subjects are uncontrolled and are treated as error. In within-subject designs, the same subjects are tested in each condition. Therefore, differences among subjects can be measured and separated from error. For example, consider the following data:
            Control   Experimental
 Subject   condition   condition   
Subject 1     12           14
Subject 2     25           28
Subject 3     29           32
Subject 4     54           57
Every subject did better in the experimental condition than in the control condition. Even though the advantage of the experimental condition is small, it is very likely real since it is very consistent across the four subjects.