Longitudinal studies allow social scientists and economists to study long-term effects in a human population.
A cohort study is a subset of the longitudinal study because it observes the effect on a specific group of people over time. Quite often, a longitudinal study is an extended case study, observing individuals over long periods, and is a purely qualitative undertaking.
The lack of quantitative data means that any observations are speculative, as with many case studies, but they allow a unique and valuable perspective on some aspects of human culture and sociology.
The original producer, Michael Apted, proposed the hypothesis that children born into a certain social class would remain entrenched in that class throughout their life.
In 1967, he selected children from the rich, poor and middle classes, and proceeded to interview and film them every seven years. The highly acclaimed series is still running, with the next set of interviews to be performed in 2011/2012, and it has provided a unique insight into the development of British culture since the 1960's.
Even this series highlights one of the major flaws of a longitudinal study, the problem that there can be no retesting or restart. Apted, with hindsight, wished that he had used more female subjects, showing the importance of the initial planning stage of a longitudinal study. Once a course of action is decided, the clock cannot be turned back, and the results must stand as tested.