Individual changes in growth, development and health can only be studied if the same individuals are measured repeatedly over a period of time. This is called a longitudinal study. However, longitudinal designs also have their constraints: No matter which design has been used, always confounding effects will occur. Three classical research designs can be discerned: (1) a cross-sectional design, (2) a time-lag design and (3) a longitudinal design (see Figure 48.1).