A within-subjects design is more sensitive than a between-subjects design because:

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Quizplus · Accepted Answer

In a within-subjects design, participants serve as their own control group, meaning there is less variance between groups (as there is only one group) and thus less error variance. This can lead to greater statistical power and a higher ability to detect subtle effects. There may be fewer degrees of freedom, but this is not the primary reason for increased sensitivity. The number of participants may vary between designs, but this is not the primary reason for increased sensitivity.

A Within-Subjects Design Is More Sensitive Than a Between-Subjects Design