What is a Longitudinal Study?

A longitudinal study is a type of observational and correlational research study that involves repeated observations of the same variables over long periods of time — often quite many years, say decades.  Longitudinal studies are also used in medicine to study uncover predictors of certain disease. The reason for this is that unlike cross-sectional studies, in which different individuals with same characteristics are compared, longitudinal studies track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations. Because of this benefit, longitudinal studies make observing changes more accurate, and they are applied in various other fields like psychology, sociology, advertising etc. Longitudinal studies observe the state of the world without manipulating it.  It has been argued that they may have less power to detect causal relationships than do experiments. But 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 by virtue of observing the temporal order of events. Longitudinal studies allow social scientists to distinguish short from long-term phenomena, such as poverty.  Types of longitudinal studies include cohort studies and panel studies. Cohort studies sample a cohort, defined as a group experiencing some event in a selected time period, and studying them at intervals through time. Panel studies sample a cross-section, and survey it at regular intervals.  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.  Many a times Operational Research Methods are used for a longitudinal study.

Some of the disadvantages of longitudinal study include the fact that it takes a lot of time and is very expensive. Therefore, it is not very convenient.