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A longitudinal study is a type of research study in which the same group of people are studied and tested at intervals over a long period of time. For example, one way to test whether a particular kind of memory declines with age is to test a group of 20-year-olds and then retest these same individuals every 10 years thereafter. A longitudinal study can provide information not only about whether memory declines with age, but when these changes first become evident, and whether there are individual variations in how fast these changes occur.

The Baltimore Longitudinal Study of Aging is an example of an ongoing longitudinal study.