When it comes to learning about the effects of aging on the body, scientists like to look for trends among large groups of people, over a long period of time, before drawing any conclusions. Such studies are called "longitudinal," because they track data over time. In other words, one population but many repeated observations.
In medical research on aging, for example, these studies can be extremely valuable. This is because they offer information about the way certain characteristics or behaviors (like smoking or exercise) affect health, disease development and mortality in people over a substantial portion of their lives.
Longitudinal studies include:
- cohort study - made up of subjects with something in common, such as where they live or when they were born
- multi-cohort study - when various subgroups within a cohort are investigated
- panel study - involves a cross-section of subjects with differing backgrounds
A drawback of longitudinal research is that these projects require significant resources of time and money to enlist and follow subjects. Such research is also observational; that is, observations are made about people without manipulating their behavior, such as which medications they take. Therefore, the conclusions must usually be restricted to which factors are linked (correlation), rather than which behavior caused a certain result (causation).
Longitudinal studies have been conducted in many different countries. Here are some examples of major longitudinal research around the world:
- Australian Longitudinal Study of Aging
- Baltimore Longitudinal Study of Aging
- Canadian Longitudinal Study on Aging
- Framingham Heart Study
- U.S. National Institute on Aging Longitudinal Studies of Aging
- Whitehall II Study (United Kingdom)
- WHO Study on Global AGEing and Adult Health
David A Grimes and Kenneth F Schulz. "Epidemiology Series. Cohort Studies: Marching Towards Outcomes." Lancet 2002; 359: 341–45.
David A Grimes and Kenneth F Schulz. "Epidemiological Series. Case-control Studies: Research in Reverse." Lancet 2002: 359: 431–34.