Cohort experiments that compare groups like the one in Table 2-2 are called longitudinal studies, since the data is collected along the natural lifespan of a customer group. By contrast, studies in which different groups of test subjects are given different experiences at the same time are called cross-sectional studies. Showing half of the visitors a blue link and half of them a green link in order to see which group is more likely to click that link is a cross-sectional study. When we’re comparing one attribute of a subject’s experience, such as link color, and assuming everything else is
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