You would use a paired samples test when the two sets of data are related, matched, or come from the same subjects measured twice. For example, it’s used in before-and-after studies (like measuring blood pressure before and after taking medicine on the same patients), or when each subject in one group is paired with a similar subject in the other group.

In contrast, an independent samples test is used when the two groups are completely unrelated, such as comparing test scores between two different classes of students.