In addition to comparing independent groups, we can also compare data across related categories, that is, categories that are not independent groups but where the observations
are related in some way as described below.
For instance, we may measure reactions to a brand before and after an advertising
campaign. If we measure these reactions on the same people, this means that each person
gives two pieces of data at two different times. The fact that each person gives data
to each time period measurement makes the data related. Time period becomes the categorical
differentiator, and the categories are related by virtue of this design.