A longitudinal survey is a correlational research study that involves repeated observations of the same variables over long periods of time, often many decades.
It is often a type of observational study, although they can also be structured as longitudinal randomized experiments.
Longitudinal studies are often used in psychology to study developmental trends across the lifespan, and in sociology to study life events throughout lifetimes or generations.
The reason for this is that unlike cross-sectional studies, in which different individuals with the same characteristics are compared, longitudinal studies track the same people and so the differences observed in those people are less likely to be the result of cultural differences across generations. Longitudinal studies thus make observing changes more accurate and are applied in various other fields.

In medicine, the design is used to uncover predictors of certain diseases.

When longitudinal studies are observational, in the sense that they observe the state of the world without manipulating it, it has been argued that they may have less power to detect causal relationships than experiments. However, because of the repeated observation at the individual level, they have more power than cross-sectional observational studies by virtue of being able to exclude time-invariant unobserved individual differences and also by virtue of observing the temporal order of events. Some of the disadvantages of longitudinal studies include the fact that they take a lot of time and are very expensive. Therefore, they are not very convenient.

Longitudinal studies allow social scientists to distinguish short-term from long-term phenomena, such as poverty. If the poverty rate is 10% at a point in time, this may mean that 10% of the population is always poor, or that the whole population experiences poverty for 10% of the time. It is impossible to conclude which of these possibilities is the case by using one-off cross-sectional studies.

Types of longitudinal studies include panel studies and cohort studies.
- Cohort studies sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period, performing a cross-section of data collection at intervals through time.
- Panel studies also use cross-sectional data and compare the same group of individuals at intervals through time, but the sample is not necessarily a cohort, as it can be a group of people who do not share a common event.

A retrospective study is a longitudinal study that looks back in time. For instance, a researcher may look up the medical records of previous years to look for a trend.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on June 14, 2016.