ABSTRACT
This chapter looks at studies that are comparisons, either of the same people as they mature over longish periods or of several sub-group samples. One of the main purposes of longitudinal studies, and of cross-sectional studies that involve age groups, is to obtain information on changes in a psychological variable over time. The longitudinal approach surmounts this difficulty since it employs repeated measures on the same group of people over a substantial period, often a number of years. In some longitudinal studies, a control group is used for comparison where the 'treatment' group is receiving some form of intervention programme. What is worth mentioning is the general size of the panels that are followed through in longitudinal studies of this kind. Occasionally, longitudinal studies even larger than those noted earlier are carried out on samples from the whole population, often children, in order to give some idea of national trends.
