To examine changes that take place over time, developmental psychologists use three different methods. In cross-sectional studies, researchers test groups of subjects of different ages. In longitudinal studies, researchers test the same subjects—usually a cohort, or group of people born during the same historical period—as they grow older. For studying adulthood, researchers sometimes use biographical or retrospective studies, in which subjects’ lives are examined backward through interviews. Each of these methods has certain advantages and disadvantages.