Longitudinal research traces the behavior of one or more participants as the participant age. Longitudinal studies assess change in behavior over time, whereas cross-sectional studies assess differences among groups of people.
For instance, consider how we might investigate intellectual development during adulthood by using a longitudinal research strategy. First, we might give an IQ test to a group of 25 year olds. We’d then come back to the same people 20 years later and retest them at age 45. Finally, we’d return to them once more when they were 65 years old and test them again.
Longitudinal research refers to repeated study of the same people over a period of time.