Longitudinal study
Overview
A longitudinal study is a research design that involves repeated observations of the same variables over short or long periods of time. It is a type of observational study. Longitudinal studies are often used in psychology, social science, epidemiology and medicine to study life span, aging and developmental changes over time. They can also be used in economics to study variables over time.
Methodology
The methodology of a longitudinal study goes beyond mere data collection. It involves observing variables over an extended period of time, often many decades. Unlike cross-sectional studies, which observe a population at a single point in time, 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.
Longitudinal studies allow social scientists and economists to study long-term effects in a real-world context. However, they are not without their drawbacks. Longitudinal studies are both expensive and time-consuming to perform. The attrition rate can be a significant challenge for longitudinal studies. Attrition rates can be high for long-term studies, with subjects becoming sick, moving away from the study area or even dying.
Types of Longitudinal Studies
There are several types of longitudinal studies, including cohort studies, panel studies, retrospective studies and prospective studies.
Cohort Studies
A cohort study is a particular form of longitudinal study that samples a cohort (a group of people who share a defining characteristic, typically those who experienced a common event in a selected period, such as birth or graduation), performing a cross-section at intervals through time. It is a type of panel study where the individuals in the panel share a common characteristic.
Panel Studies
A panel study is a longitudinal study where data is gathered for the same subjects repeatedly over a period of time. Panel studies are the best method to study changes in the same individuals, but are costly and time consuming.
Retrospective Studies
A retrospective study uses existing data that have been recorded for reasons other than research. A retrospective case series is the description of a group of cases with a new or unusual disease or treatment. With a retrospective cohort study, the group of interest are individuals with a certain exposure or treatment, the cohorts are defined, and the investigation proceeds backwards in time to assess how many subjects in each cohort have the outcome of interest.
Prospective Studies
A prospective study watches for outcomes, such as the development of a disease, during the study period and relates this to other factors such as suspected risk or protection factor(s). The study usually involves taking a cohort of subjects and watching them over a long period.
Advantages and Disadvantages
Longitudinal studies have several advantages and disadvantages.
Advantages
Longitudinal studies allow researchers to detect developments or changes in the characteristics of the target population at both the group and the individual level. The key here is that longitudinal studies extend beyond a single moment in time. As a result, they can establish sequences of events.
In a longitudinal study, researchers are able to look at the impacts of specific differences in variables over time. This is a major advantage of longitudinal studies, particularly for researchers in the fields of psychology and social science.
Disadvantages
However, longitudinal studies also come with significant disadvantages. For one, they take a lot of time. They are not suitable for studies that require quick results.
Another disadvantage is participant attrition. This refers to the dropout rate of participants over the course of the study. The longer the study, the higher the attrition rate. This can lead to bias if the attrition is not random, and can be a significant limitation.
Applications
Longitudinal studies have wide applications. They are frequently used in psychology to study developmental trends across the life span, and in sociology to study life events throughout lifetimes or generations. The reason for this is that unlike cross-sectional studies, longitudinal studies track the same people, so the differences observed in those people are less likely to be the result of cultural differences across generations.
Longitudinal studies are also used in medicine to uncover predictors of certain diseases. In communication studies they help identify the signals of the communication. Longitudinal studies in business are needed to understand consumer behavior and to study changes in market trends.