Longitudinal. Like age-based cross-sectional methods, this approach is designed to provide data on how people change over time. However, it overcomes the methodological problems of cross-sectional studies by making repeated measures on the same group of people over long periods of time. This approach also allows researchers to make detailed observations of major points of change in people's lives (if the times between observations are relatively short). This research would typically take place over a period of some years (or even decades). Although it is a very powerful way of looking at age-related changes, it clearly also involves a great commitment in time on behalf of the researchers (and willingness on the part of their participants to continue participating in the research). Perhaps the best known longitudinal studies are cohort studies that aim to follow people born in one week from birth throughout their lives.