Advantages of Within-Subject Designs (2 of 2)

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The important point is that this small but consistent difference can be detected in the face of large overall differences among the subjects. Indeed, the difference between conditions is very small relative to the differences among subjects. It is because the conditions can be compared within each of the subjects that allows the small difference to be apparent. Differences between subjects are taken into account and are therefore not error.

Removing variance due to differences between subjects from the error variance greatly increases the power of significance tests. Therefore, within-subjects designs are almost always more powerful than between-subject designs. Since power is such an important consideration in the design of experiments, within-subject designs are generally preferable to between-subject designs.


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