The methods used to study development are as varied as the theoretical viewpoints on the process itself. In fact, often (but surely not always) the researcher's theoretical viewpoint determines the method used, and the method used usually reflects the question of interest. Age correlates with all developmental changes but poorly explains them. Nonetheless, it is often a primary variable of concern in developmental studies. Hence, the two traditional research designs, longitudinal methods, which examine one group of people (such as people born in a given year), following and reexamining them at several points in time (such as in 2000, 2005, and 2010), and cross-sectional designs, which examine more than one group of people (of different ages) at one point in time. For example, a study ...